Trump wins if he wants it. His age, senility, scandals, none of it will matter. Instead he will have a lock on a significant fraction of the electorate who remember that things were better in the 4 years of Trump's term than they will be in the 4 years of Biden's - including any possibly conflicts in addition to the austerity that will continue.

If Trump doesn't want it, it's hard to say who would step up. De Santis kind of has Trump energy, but it's hard to capture the celebrity of Trump and the completely blank ideology of Trumpism that can take any policy you throw at it - anti-forever wars but also we should bomb Syria and China, protectionism combined with free trade, etc. You'd have to be Mike Lindell basically for the next Trump, some guy that was a prominent Trump supporter but outside the GOP elite while maintaining the complete lack of ideology that Trump did. But the actual Mike Lindell is apparently going for governor of Minnesota, so who the fuck else would it be. Otherwise, chances are it's just gonna be some vat-grown GOP candidate. I've heard the Rock might interested in some presidential run but he supported Biden, so he'd be unlikely to run against him. The Rock has the celebrity and the complete lack of ideology that Trump did so he'd be another good candidate, he's just not nearly as dumb as Trump or Lindell.

If Biden doesn't run, Harris seems like the logical replacement and potentially the Dem machine clears the way for her (no way Bernie tries another run and there's no one in the field that could capture Bernie's crowd and energy). If the machine doesn't, Harris is hilariously unpopular and could lose to Booker or something even after being VP. But no one else has that Obama magic glamour to them that Biden did, it's hard to say what the Dems would do if Biden doesn't go for it. Harris would absolutely lose to any GOP candidate.

If Trump and Biden go at it in 2024, Biden loses. Not by popular vote, but Trump just needs to get a smallish percentage more in a few key states compared to 2020 and he wins. 4 years of austerity and 2 years of legislative deadlock will mean the US is just as fucked as it was between 2014 and 2016, fertile ground for Trump's win.

The GOP take back the house and senate, the Dems would have to outpace the best performance of the president's party out of the last, like, 70 years to have a shot at maintaining the majority in both. No way in fuck do they manage that, absent something like 9/11 (the 2002 midterms were the best election performance). The majority is so thin and the margins are so thin for so many congresspeople and senators that the Dems are completely fucked in the midterms. Even if the mild stimulus Biden gives still has an afterglow by the midterms, it isn't going to be enough.

To be clear though, Biden and the Dems could cement a majority and the White house for 2022 to 2028 by just passing stimulus, repealing filibuster, admitting PR and DC, and doing a couple more populist policies like M4A or cancelling student debt and decriminalizing marijuana or something. They're not going to, but it's completely within their grasp and they will choose for the next 4 years to not do the things that will ensure their victories and blame the voters.