All they are are DPS races.
it still feels like all your doing is taking advantage of your invincibility frames after getting hit
If this was actually the case in the game's design then the boss rush at the end of the game and its limited health recovery would be impossible if you ran out of weapon ammo and E-Tanks.
The boss battles are the biggest appeal of Mega Man because they require the most effort to defeat of the game's challenges, give the cast personality, and they facilitate the concept of growing stronger through the course of the game. You start with eight that you can pick at any time and in all likelihood you're going to get stomped by their stages that first play. You're not going to make it through without learning through repetition, and those techniques you learn by doing so are often required to beat the boss. After eventually beating all eight with each one requiring multiple attempts to learn and grow, you're tasked with beating all of them again in a row at the end. Being able to defeat all of them in this restrictive setting shows how far you have progressed.
All the bosses' attacks are avoidable but they are meant to be hard not only because of the NES difficulty mentality but it makes them more memorable. They aren't nameless enemies like "cat on slot machine" from Rescue Rangers or "robot with jetpack" from TMNT that is only named in the manual. These bosses have clear faces, names, and minute hints of personality in how they fight and pose. The boss weakness chain was created to keep the player, often young children, from getting discouraged by the difficulty. Instead of it being this vague and condescending concept of "git gud", there is now a hope and a goal. If they can just find out what boss is weak to this one weapon they got, they can progress. And that's not even including the abundant amount of E-Tanks available to refill health at a time of need.
The bosses are difficult to defeat outright requiring effort to learn and adapt, while still offering alternative solutions for players who don't have that time investment or skill. This is exactly what is considered great game design that appeals to a wide range of players and skill levels. Instead, you're taking the worst of both attitudes. You're not learning how to fight the bosses claiming they're unbeatable without abusing invincibility and are complaining they're too easy when you use the systems meant specifically to aid lower skilled players who are having trouble fighting bosses.
But let's say for the sake of a terrible argument that Mega Man 2's bosses are bad because they can't be beaten without abusing invincibility frames and the weapon weaknesses trivialize fights, Mega Man 3 and beyond introduced the slide mechanic and X / Zero titles use dashes and wall jumps. You now have tools specifically for evasion and increased movement that rewards skillful dexterity and reflexes, essential to fast-paced and satisfying action gameplay. And in later Mega Man titles the boss weakness deal considerably less damage like two or three pips of health instead of Mega Man 2's massive amounts of damage, so even with the weakness you still have to put some effort into the fight that no longer ends in 4-5 hits like you argued in your OP.
I will concede that the security laser / Wily Alien bosses that can only be beaten by Crash Bombs / Bubble Lead are bullshit since you'll most likely need to game over to recover weapon energy for it if you fail, but that and your post only apply to Mega Man 2, so why did you make blanket statements about the ENTIRE franchise in your OP and particularly in your thread title? Seeing as how you got me to click your thread, I guess I have the answer to that, don't I?
On a different note, I'm surprised no one has mentioned the "Megan robot masters" typo.