Next Democratic president won't be until 2037.
So you think Biden wins re-election (serving until 2029) and then the President after him is a Republican who serves from 2029-2037?
More likely we get 12 years of DeSantis/his VP in 2025-37
Why would you even think such a thing? The GOP would be hard-pressed to win a single election. Even with OH, FL and IA locked down - and FL isn't - they need to flip GA, AZ and WI to win, and hold NC and TX. They might be able to achieve all that in one good year for the GOP, but there's a 99% chance they can't pull it for 3 elections in a row (even 2 elections in a row is somewhat unlikely).
Anyway, the answer's Harris. She's Biden's natural successor. If Biden dies in office, Harris obviously becomes president, and if Biden retires (whether in 2025 or 2029), she'd be the Democratic Heir to the Throne, and because of the reasons aforementioned, she'd then be the favorite to win (since the national electorate is no longer split down the middle but tilting or possibly even leaning Democratic).
I mean if DeSantis has really high approvals it could happen if his VP is the nominee though I doubt it as even Gore lost 2000 with a Dem President at 67% approvals.
DeSantis most likely would get reelected to as incumbents do indeed get reelected just like Trump would have been without COVID.