I think the "she wasn't selected by the people" route is a dead end.
We almost saw a lunatic assassinate President Trump who was selected by the GOP voters. If he'd been incapacitated, would the GOP not be able to replace him? If he just decided after the attempt, I'm not going to risk my life, would the GOP be able to replace him?
If Trump decides: screw this, I'm going to play golf, run my business, and write another book, should the GOP be able to replace him?
It's just a dead end. Of course Biden is old, VERY VERY questionable in the office now, and shouldn't have ever even been elected..... no argument..... but of course a candidate can be replaced if they drop (or get forced by a bullet or their party) to leave the race.