President Biden understands the stakes even if the legacy media is unwilling to say them out loud. He yields NOTHING or loses EVERYTHING. Post-debate there is no in between.
If he withdraws voluntarily as the nominee, in short order he would have to resign as POTUS or be removed via the 25th amendment. The idea that he could be the lamest of ducks through January 2025 even as the new nominee rallies the entire legacy media to trash him (as would be necessary for there to be any chance of getting elected) is not sufficiently plausible to make it worth it to Biden to take that risk.
Sooner rather than later, Biden will be threatened with the 25th Amendment and will be told that if he resigns ahead of this then his successor, POTUS KHarris, will pardon Hunter. This will be the enticement.
It is implausible to me that the Dem elites will pull up short of using the 25th Amendment if this is the only way to resolve the issue.