I say run him anyways - he can be the token ‘old white dude’, with AOC as his vice. If he kicks the bucket or finds that he is breaking down, he can pass the baton. Otherwise, we get an extra four years of progress for each term he runs. If dementia is a concern, just write a public contract under what terms he is out of the running.
Something like “We know I am getting old. AOC has my right of attorney for determining my mental state.”
There will be. It is just a question of how many presidents there are.