Biden will stay for now because of number of reasons:
pushing him via 25th amendment, donor coup etc. would look extremely bad for Democrats;
longer and more orderly transition towards Kamala presidency, regardless whether she stays until January 2025 or actually wins election
practical issues related to campaign finances, which are to be inherited by Kamala rather than bitterly fought over by feuding candidates
Controlled and easily manipulated media interview would achieve some of those aims, at least temporarily.
That is all true although the donors have a lot of pull. If they pull back, it is going to make it tough.