If Biden steps down, his candidate successor will have a hard time, as being lesser known. (S)he will need time to "build up". That is clearly a disadvantage, and probably it is the strong argument for the Democratic Party of the USA to keep him in place.
Replacing him is a possibility, at least on the level of open communication of the Democratic Party, but as time is going away, so will have the successor lesser and lesser chance to win over Trump. In this matter, time works for Biden.
What is the point, where we will be able to say with a high probability, that Biden will remain?