Both candidates lose to a generic 3rd candidate. That was true even before the debate. The problem is finding a generic 3rd candidate that people agree on.
Truth is, the only person I can think of with enough name recognition that might bring enough support from progressives, moderates, and centrists this late is maybe Pete Buttigieg. But I wouldn’t bet on it. Biden needs to use every moment from here until November to get in front of people to show that the debate was an outlier.