There's no time to step aside. Primaries are over. Trying to squeeze primaries in now would be a disaster.
The time for Biden to step aside was a year ago. And honestly, as much as I have liked his policy (excluding Isreal), and still trust his administration to execute on even if he's incapable of leading it himself, that debate was terrible for his campaign.
There are only two options now. Continue running himself and try to salvage the situation, or hand the reigns to Kamala. Neither is a good choice, campaign wise.