Republican voters are a lot more loyal than you think, they'll choose a 'moderate' Republican over an election denier, but a Democrat is a different story. I think a Biden/DeSantis election would be worryingly close, I can see a scenario where the center-right deludes themselves into voting for DeSantis.
A moderate Republican could be a tremendous challenge to Biden if there isn't a clear and visible post-Trump/covid recovery of the economy by 2024, which I think is on track.
DeSantis is not as strong outside of right wing politics as Republican voters would like to think. He's like Trump in a lot of ways, particularly he's an ego maniac who relies entirely on bigotry to win and refuses to step outside his echo chamber of redneck Floridians, Trump supporters, police families, Fox News, and other conservative coalitions. Like Trump he demands to be praised as an adored leader without engaging with anyone besides his "base" who he knows will do it for cameras.
I don't see the guy performing well on a stage with an incumbent President who has real accomplishments and results to tout when all he got to show for is a bunch of bigoted hate towards gay and lesbian people, "woke mob" fear mongering, hating on Disney, hating on the media, hating on teachers. All this while getting roasted by Trump day after day and probably going with some wishy washy "I supported Trump and won't attack him" nonsense, showing any potential moderate voter that this man is still a stooge for the guy and will govern just like him.
DeSantis is a troll and a clown who's every bit as lost without zaddy Trump and his failed white supremacist movement around anymore as his racist peckerwood voting base who refuse to pull back their extremism, see "America" doesnt revolve around them and their hatred, and dont recognize that "owning the libs" is no longer a great campaign promise after what we saw with Trump. It doesn't seem like the Russian troll farms are batting for him like that. He will be a polarizing figure among Republicans who still support Trump. Trump will not concede his campaign, inevitably obstruct DeSantis's campaign, rally hardcore conservatives against him, and spiral the party into infighting chaos.
I just don't see it for the guy or his party next year with the direction they're going in. With that said Democrats can't sleep for a second, they have to deliver for the next year and they have to be prepared for a tight race and an ignorant public.