If the convention was a week or two away - Biden could easily ignore the reps asking him to resign and weather the storm. The problem is the convention is - I dunno - ~6 weeks away. The stats could be really bad by then. And the he has so many other problems. The majority of his supporters want him to step down, etc. If he can avoid "bad days" until the convention is over - he could easily survive but look like a shit candidate because all the trend lines continue to look horrible if not nightmarish for him. If he gets the nomination - even a single bad day so close to the election could make even his diehard cheerleading supporters see he's doomed.