Will there be video evidence strongly implying that Joe Biden regrets dropping out of the presidential race, before the end of his term?

Some things that would make this resolve YES:

Biden says he would have won had he been the nominee

Biden says it was a mistake to drop out

If nothing like this comes out, resolves NO at the end of Biden's term.

Things that wouldn't count (not direct enough):

Biden says top Democrats shouldn't have put pressure on him to drop out

Biden says he wasn't too old to run

I will not bet here.