Yes, Trump is extremely racist. He's also a right-winger, so that's a given. The distinction between someone horrible like Trump and his opposition should be that the opposition is not horribly racist and fascist and doesn't do things like fund a genocide.
I don't vote for Democrats because of their label, I vote based on their potential to actually be meaningfully better. Biden is indistinguishable from your rank and file Republican, and while that's "slightly less bad" than Trump himself, it's not good enough. I am not a Republican, and I will not vote for representatives who share so many of their views. Biden needs to go, he's going to cost the Democrats the election.
This is the way the DNC set their rules up, they've been ok using the system to kneecap progressives, I see no reason that they shouldn't do that to Biden. I'm not precious about the DNC and I have no illusion that it's democratic, so they just need to stop pretending they're being held back by principles and just pull the levers they always pull to control the convention outcome.