Dying empires throughout history have, in their final years, tended to be led by old, weak and mentally infirm men. Eg Soviet Union, Ottoman Empire, Han Dynasty etc etc
It's uncanny, like the empire in question is expressing its death throes in its leader.
Unlikely Biden will step down although there are a number of mechanisms in place. It would have to be before the Democrat Convention as they are so stage managed that no way would there be a revolution. No sign that Harris is looking to evoke the 25th Amendment, although she could, if she and the senior Democrats really cared about their country.
Anyway if there is a surprise and Biden does the RIGHT thing for America, then I think Gavin Newsom should be the candidate. Governor of true blue California, good record, excellent communicator, looks presidential compared with Trump. Best of all, he has all his marbles.