The only time in recent decades that an incumbent President did not run or stepped away from running was 1968 (Johnson) and the Democrats got beaten in the election.
Obviously, there were many reasons why that happened beyond switching from the incumbent to a new candidate, but Johnson also announced the decision on March 31st of that year.
It is now June 28th, roughly 4 months away from the election and ~50 days from the national convention. IMO it is too late for Biden to step aside and the Democratic party to get a new candidate up & running and, most importantly, on the ballots of the states.
And, as has been asked many times before, what vastly better Democratic candidate exists that could get elected?
Gavin Newsom would get beaten by Trump IMO; he is from CA, has been a controversial governor, he is more liberal than Biden and does not have Biden's record of achievement and experience to run on.
Mayor Pete would get crushed by Trump.
Harris is really the only one that could carry the mantel, but she is an even more flawed candidate IMO than Biden.