I'm not an American, but I am very much a Sanders supporter (and would very happily vote for Warren if she became the candidate). Biden is way down on my list of people I would like to support. Here's my attempt to sell you on Joe Biden anyways.
Let's assume that the bare minimum President Biden gives the United States is a rollback of the most egregious Republican policies of the last few years: no more kids in cages, lifting the immigration bans, maybe beginning to restore the bureaucracy that got hollowed out (like the State Department). Pretend he does absolutely nothing else during that time, enacts no policies of his own, doesn't try to push a single thing beyond what already exists. Short of a major accident, health issue or assassination, Joe Biden will be President for the next four years, and even if he does die in that time, the Presidency will be in the hands of a Democrat.
In this scenario, Joe Biden is still giving you the gift of time.
Two years of Obama-era immigration policy, while certainly not perfect and definitely not a deportation-free policy, means two years to prepare for the possibility of it happening again, to build organizations and communities better able to protect against its people being stolen by ICE and imprisoned for no good reason. Two years of building up the country's civil service again can mean two years of reinforcing something we now know can be lost so easily due to neglect or passive malignancy. Two years of protection for the Affordable Care Act means two more years that Americans will have slightly less shitty healthcare than they could've had under Trump. That's thousands of people that get to be alive. Two years of a Biden administration directing Supreme Court appointees... well, maybe they'll all be Merrick Garland'd, but at least we know what that looks like, and at minimum we won't be filling those lifetime appointments with right-leaning justices in the meantime.
Even if you assume that after two years Biden's administration will be so weak that Democrats lose the House and Senate and then the Presidency in 2024, Biden will have given you time that you do not currently have. Four more years of Trump guarantees that he will move forward on his agenda, emboldened by a renewed mandate and either unshackled because he no longer has to worry about re-election due to term limits, or unshackled because he no longer has to worry about term limits. And yes, this is partially a "he's not Trump" argument. But let's be clear about what that means. It's not "well we got essentially the same guy except he doesn't spray tan himself." There is a meaningful difference. More importantly, it means the activist communities that have formed around people like Bernie Sanders and Elizabeth Warren can continue gathering resources and planning for the future. Building coalitions, pushing downballot races further left, keeping Medicare For All and corporate malfeasance in the spotlight--all these things are easier to do when you're not simultaneously busy trying to fight the existential threat that is the current administration.
The biggest mistake we made as a society in 2008 is that we thought the job was done once Obama won. We were wrong. Some people, who have suffered longer and harder than most of us, knew this implicitly and tried to make us see the error of our ways. The job is never done, victory is never complete. Even if it accomplishes absolutely nothing else of value, a Biden administration buys you time, and that can be the most valuable commodity of all.
I don't want to take away from an otherwise great post, but I fundamentally disagree with this premise. I can't say I share the same optimism for the scenario you presented, both as an American, a minority, someone with health issues and a liberal. I will hold my nose and vote for Biden as I did with HIllary if Bernie isn't the candidate, although I always believed that it was going to be a long shot.
The perfect start for my argument is the scenario of a Biden administration. There would be some immediate benefits, including a rollback of the most egregious policies pushed by the Trump administration. The ones that I can openly agree with are the change in Border Patrol directives (although Biden still leaves in place a horrible organization), immigration bans, and I can somewhat concede bureaucratic changes in the EPA, Department of Education, State, Defense, etc. The problem starting with all of these things is that a return back to the end of 2016 is not a great point to start. Many of the same issues Obama left behind are still there, including a healthcare system teteering on irrelevancy from the SCOTUS, a tax system that benefits the wealthy, large portions of the country barely doing okay and staying above water, a middling foreign policy legacy now in rubble under Trump, political and social division, crumbling infrastructure, a vulnerable elections system ripe for future manipulation by foreign powers, and a litany of other issues. None of these things are anything to be optimistic about, and those shortcomings that persisted under Obama only worsened the reaction that created Trump. Its the prime reason I have no confidence in your next statement, that Biden will give us "the gift of time."
People are being either overly optimistic or naive if they believe the white supremacy or populism of Trump will leave after he loses in 2020, if that happens. While the Democratic Party were losing left and right under Obama, the Republican Party had a resurgence and got numerous governors, local officials, and members of Congress elected. The rather poor field of 2016 and the moderate recovery we had for the 2020 Democratic primary was because of how decimated we were under Obama. Under Biden, we will have young, "well-spoken", White male, Ivy league educated, and military veterans looking to attack an old Biden in 2024. These include Tom Cotton, Todd Young, Josh Hawley, James Lankford, Ben Sasse, and a litany of other Republican governors who will run a hard campaign both bringing the tenets of Trump with a smoother dog whistle. The reason why I am rather pessimistic is that your argument only matters if we maximize time. Biden is not going to push for sweeping changes that will disrupt the status quo on banking, taxation, health care, education, the environment, or other issues that Bernie pushed for. Granted, it didn't mean that Bernie was going to be successful in pushing legislation, but he would have maximized his time to use his powers as President, the same way Bush, Obama and Trump pushed the envelope of the Executive Branch. Neither Bernie nor Biden will have a second term, and we need to maximize that time with someone who will push the margins of the Presidency to the left while we have time to. The only immediately and uncontested benefit of a Biden Administration will be the likely two open SCOTUS seats that can open up. That is the only reason I'm even bothering with Biden.
The two years of Obama-era immigration policy ultimately didn't matter because Trump was able to largely do away with it, and push the template for his now right-wing Border Patrol agency. To say nothing of his mass deportation that tore families apart, including several that I knew of personally. I don't want another Biden to simply pull the margins back to the center for another Republican to pull it further and further right. Obama had his barriers and shortcomings, but one of his biggest failings was not maximizing the Executive Branch to push the envelope to the left. We are not going to build up our immigration system or civil service in two years without a dramatic vision and an aggressive will to pull things around. I don't have any faith in Biden doing that. Two more years of the ACA will be lifesaving for millions like myself who need it, but will still leave a fundamentally vulnerable and flawed healthcare system that has been successfully clipped by Republicans, the SCOTUS and lower courts. Biden is not going to push for something more dramatic, like an actual replacement of the whole damn bill and a further conversation about how all these measure are taping over a broken system. Why would I ever be enthusiastic about a system like that? Why would I ever have hope in a future like that? Democrats are the only sensible players in town, but they only succeed in pulling us to the center on most issues, for an inevitable Republican to pull us further to the right than we did before. Smashing what limited progress we made in the first place. If the United States is a person drowning beneath water, Republicans are pulling us further beneath the water, and Democrats are making limited pushes to the surface, but will never get us above the water. This is not a "both sides" justification, but we have to admit that too many institutions are broken in this nation, and incrementalism isn't working in our favor as a nation and as a people.
The added slap in the face is toward your point on activist communities. I worked for several, and its often extremely difficult to get people to be motivated in a system that they believe will always weigh against them. Biden would be a better alternative, but many of us are already wary of a 2024 resurgent Republican that now has the Trump mantra with him. He's going to change the party in the same way that we saw with Reagan, and many of us have no optimism to continue preserving the same foundation that often left us in the gutter in the first place. It's a big reason behind why socialism is "en vogue" with so many young people. The Democratic Party is facing a major issue over its identity, and I don't know if those issues are going to be placated with a contentious convention and the possibility of a Biden Administration. I'm going to guess that many of these people are again going to be left disappointed, and feeling as if their voices aren't being recognized and implemented. I don't even want to see a party if we lose again this November. We will still have the resources toward building coalitions for a more progressive agenda, but the continual barriers and intersections of Wall Street, class, region, race, government and other variables will continue to be walls. I'm going to be afraid that younger people in future years will be less willing to work within the system instead of just tearing the whole thing down.
Your final point is well-taken, and I agree with it. Our job is never done, but we also didn't leave a strong foundation for our work to endure and continue. We did a terrible job of making sure our policies and agenda would show to be beneficial to many people who suffered and were fed up. A Biden Administration doesn't buy us time. Not in this political climate. It just kicks a can down the road toward our court, a slight respite from sinking further into the abyss but not a future.