We are undoubtedly stuck with Biden unless he dies suddenly. A late change of heart isn't his style, and he's got an ego to protect.
VP Kamala Harris is the obvious successor but I don't get the impression the leadership likes her very much and she's been mostly sidelined for 4 years.
Unless he dies, too, Trump isn't going away anytime soon either. So aside from COVID, it looks like a rehash of 2020, only Biden's standing is weaker and Trump's negatives apparently aren't as salient to voters.