Well, I mean, for starters we just elected a guy who was a bit more of a celeb than a legit contender. But mostly it's just my gut. Which admittedly is often wrong in politics. Like my gut told me Trump would last about as long in the 2016 race as Bloomberg did in 2020.
I think one of Biden's big troubles is that he doesn't contrast enough with Trump's personality flaws. Trump is bombastic and mean in how he interacts with the media and others. Biden isn't that different. He shouts at people during townhalls, etc. Trump is a shameless womanizer; Biden is handsy. Trump is incoherent. Biden is worse. Trump is getting very old. Biden doesn't seem to be aging well. A middle of the road voter who is suffering from covfefe-fatigue isn't likely to see Biden as a big improvement imo.
Michelle is the opposite. Nice, decent, clear-minded. Speaks in complete sentences. No personal scandals. If the feeling of the overall intensity and constant friction of the Trump years is wearing on voters, Michelle would be a great change of speed.