Just stepping down implies a "failure" to some degree. And if Harris is bypassed for some reason, THAT TOO would be perceived as some form of "failure" (in Biden's judgement in picking her b/c she isn't up to the job.)
Personally, I have no problem with Harris becoming #Potus if something should happen to Biden. If she were the nominee, I'd vote for her, so why does Biden's age matter?
@uspolitics As expected our top “choices” are two people who faired poorly when they ran for the nomination on their own. But anyone paying attention knew they’d been preselected for the role of future nominee.