Biden is just as likely to win/lose as Kamala is, as is Bernie, and so on.

Thanks to ballot access deadlines in several key states, this just isn't the case. It's Biden or Trump unless one of them dies in the meantime. If you tried to field Kamala as a replacement against a living Biden, you'd end up with a number of electors under faithless elector laws required to vote for Biden despite him having stepped down to make way for Kamala, possibly enough to trigger a one vote per state decision.

I’m still of the opinion that Trump has gone way too far and there will be so many people coming out to vote against him, that he’s essentially running against himself.

You aren't wrong about him essentially running against himself. It's entirely about whether or not people who would vote against Trump actually go out and vote for Biden (or his replacement should he die) in sufficient numbers.

But it's nearly always the GOP running against themselves, turnout is basically what decides US presidential elections. It's just writ lager with Trump.