Although more a problem for Biden ––
Agreeing to debate that early was an unforced error by Trump. It was bravado in response to Biden’s bravado.
Self preservation should have told Biden not to do anything that would expose him one to one so obviously prior to the nomination. He may have thought it was the thing to do, prove his competency and energy, maybe head off any thought of a coup at the Democrat convention — but it was his dementia talking. That Pelosi and other high-profile Dems were taken aback is telling that it was not a put up job to maneuver Biden out even if some think it has a silver lining.
Trump should have helped his enemies (the entire Demonrat Party) destroy themselves by NOT helping with an exit plan or inducement necessitating one by exposing how bad off Biden is in time for their convention. Trump should have said I’ll be happy to debate you the day after your nomination. Now I happen to feel Trump is not Republicans best candidate and that given the weakness of Biden we have foregone an enhanced opportunity to win with a superior executive not hampered, distracted and with a better grounding for these trying times. However beating Joe Biden is doable by Trump pretty much no matter what, Biden is that vulnerable.
One ameliorating thing is that the Democrats do not have a strong bench although they think they can drag anyone across the finish line against a Trump, indeed that their machine can beat anyone. I do not think so. Then again the Dems also realize that Trump has a propensity to say stupid things, the proverbial eat a live rat on stage. That is a true possibility with him. Nevertheless Trump benefits from the worst contrast the Democrats present, whether Biden or Harris or any of several other icky Dems who may be drug out from the wings (not including Moochelle Obama because I still believe she is not available). Biden is the easiest to contrast even with a gaff by Trump.