Bidens age was always a big issue. He had a low bar to clear at the debate. He was not able to finish a sentence so proved all of the right wing media's accusations of him.
Those who may have been on the fence, saw a weak old man. In America, it feels like weakness is the worst traint - worse than dishonesty or criminality. That debate is etched into peoples minds.
Biden has made it a referendum on him and his age, which is a losing position.
And now with Trump surviving an assassination attempt, he can play the sympathy card, take the moral high ground and make it look like it is the left that incites violence. It will be a very hard line for the media to walk now because Trump will milk every last drop and the media will find it hard to push back without inadvertently putting the message across that "he had it coming".
I don't think another Democrat will even want to step up against a man who cheated death.