A few thoughts.
1) I do not think that Biden will be the candidate. I believe Harris will be.
2) The results depend on too many factors to really discuss simply.
2a) One of those factors is the Republicans. The longer they hold on to this insane CT the election was stolen nonsense the less likely they will be competitive in either 2022 or 2024.
3) Harris would be a weak candidate if the Republicans can finally unite. But fractured and feeding on their own insures that the weak candidate wins.
1) I don't see any reason why Biden doesn't run in 2024, unless there is a change in his health. Yes, he's in his late 70's. So is Roger Waters from Pink Floyd, and that guy goes on world-wide rock tours. He gets the best health care in the world. (Biden, not Waters).
2) Actually, I don't think there will be as many factors are you think. The re-election of a president often comes down to "Are you better off than you were four years ago?" Obama, Clinton and Reagan could answer that question in the affirmative. That's why they won second terms. Trump, Bush-41, and Carter couldn't answer that question in the affirmative. That's why they lost. Regardless of where the future takes us, given what an absolute shit-show 2020 was, any president can role into 2024 and say, "Yup, we are better off."
2a) I agree that with the GOP the inmates have taken over the asylum. I think this happened long before Trump came along. I think it started when Obama got elected, and the GOP put "black man in the White House" somewhere between "Black man dating my daughter" and "black man in my kitchen at 3 AM" I don't think they are going to get any saner any time soon.
3) The GOP has pretty consistently gotten between 45-47% of the vote since 2008. I don't think those numbers are going to change no matter who the Democrats put up.
The real problem they have is that they are still terrified of Trump and are willing to appease his every whim. Even if Trump isn't the candidate in 2024, he'll cast a long shadow over the GOP.
And again, I think Biden will run again and win.