Of course it’s not too late. I mean, given his age there’s a non-trivial chance he won’t even be alive at the nomination convention, and a marginally bigger one he won’t be on election day and you would have to replace him then. So it’s obviously possible.
It is likely to be politically difficult, but after that car crash performance last night, I am seriously beginning to doubt whether he is electable even against the Orange one and the Democratic party owes it to itself, to the people of the United States, and to the people of the world to put up a credible candidate who can beat Trump.