Of course there will be those who voted for Biden in 2020 who won't vote for him in 2024. LIke all second-term elections, this will be in large measure a referendum on the incumbent. I know the Dems want this to be a referendum on the prior President, but that's not how most will approach this election. The problems that Americans face, and the fears they confront, in 2024 are not of Trump's making - how could they be? Biden is weak in ways similar to how Carter was weak, with the added burden of being perceived, rightly or wrongly, as suffering from senility and lack of vigor.
I was in Charleston on Saturday for my nephew's wedding, and I spoke to more than few folks who agree with me that "neither of the above" is the choice they will make, with a lot of support - strong support - expressed for RFK, Jr. Mark my words - the best investment Democrats can make is pull out every trick in the book to deny the voters that third alternative. Democracy is the what the Democrats need to fear most.