but the people you're talking about here (clinton/republican voters) aren't people that "went against her" - they're people that went for her, but didn't feel comfortable voting for downticket democrats. I don't know if I buy the idea that those people will now vote biden/democrat under the idea that "if trump still wins, we'll have divided government, which I like" - that's voting for unified government in the hopes that you get divided government, and I don't really think it tracks
it's possible that these people show up, and it's possible that they vote straight democrat, but I don't think many arguments outside of "they think that not just trump but the republican party as a whole have completely fucked up governance" really explain a full-party switch
Because many of the people who split their tickets between Clinton and Congressional Republicans
did not like Clinton. They were voting against Trump and wanted to make sure if Clinton got in (which seemed like an inevitability), she would have a Republican Congress keeping her in line.
Just as a personal example - my home district, Minnesota's 3rd. Fairly well off, educated suburban district. Voted for Clinton by 9 points. Our incumbent Republican rep, Erik Paulsen coasted to re-election, winning by 14 points. Fast-forward two years and Paulsen gets blown the fuck out, losing by more than 11 points to Dean Phillips (an evil moderate Democrat). Larry Sabato now rates the seat Safe Democrat for 2020. Many of those gains are locked in.
Like I said, ticket splitters in 2016 were voting defensively. They found Clinton to be repulsive, but inevitable and Trump worse, so they voted for split government. In 2020, they don't hate Biden but still hate Trump. Unity government under Biden or split government under Trump are both preferable outcomes to unity government under Trump (which is not even something they assumed was possible in 2016). So the best way to ensure either of those outcomes happen is simply by going D down the ticket. "President Trump" was not at the forefront of their minds in 2016 - it was in 2018, and it will be in 2020.
I could be totally off the mark here. But that is my theory. There is a reason most forecasters, pundits etc. are assuming that even if Trump wins, Democrats will at least hold the House majority. Ticket splitting will probably work against Trump this time, if anything.