You're forgetting about the Republican party's limitless ability to engage in hypocrisy. (Remember, the party that contemned Clinton for sexual misconduct now doesn't care about such things when Trump did them.) I'm sure if Biden became the candidate, any time he sneezes it would result in hours of coverage on Fox news suggesting that he's at death's door and that we need to stick with Trump because his doc said he was so healthy.
Sadly, given the underlying racism that seems to be evident in a large part of the electorate, "old white men" may be the best way to beat Trump.