His chin was his biggest weakness many ways, he has almost zero defence, he plodded forward taking shot after shot. He used his chin as his defence, but like many fighters before him there comes a time that your chin can't take any more. We saw a glimpse of it in that sparring video against Usyk, Allen himself said he was snoring standing up. Imagine what we haven't seen.. because that shot from Usyk wasn't exactly huge, then combine that with Hearn almost encouraging him to retire, Hearn knew it was time for Dave to call it a day.