I personally think Biden would have no choice to resign. How can go to the American people and say "I guess I can't do this job anymore", but still do it for a few more months?
It would have been different had he come to that conclusion 12 months ago, and allowed a contested primary for his replacement, but he didn't. The only firm conclusion to come out of the Primaries is that most democrats want the incumbant. So, give Harris the benefit of incumbency.
It's the simplest path to make the convention not a shitshow. And remember that they need to settle this before the convention, because of the registration deadlines in Ohio and other states. If they leave this to rhe convention, there a real chance the Democrat won't be on the ballot in Ohio.