How's Biden's health? By any objective measure he's a very old and fairly frail old man. The best campaign strategy for him is to do what he did last time, which was essentially to not campaign at all.

That worked fine against Trump last time because Trump is the living example of Never interfere with the enemy when he is in the process of destroying himself (Napoleon, I believe). But what if it's DeSantis or Nikki Haley he's up against? He'll have to hit the road and do some actual campaigning...

So what?

The reason Biden will be the candidate isn't that he lacks any possible flaws. Or even that he's the best candidate. He'll be the candidate because he's the incumbent President and no one's running against him with a chance of winning.

There's no magical person behind the curtain that has veto power over the primary results. Biden will get the most delegates because there's no other credible person on the ballot. Once he has the most delegates, he'll be the nominee because that's what the DNC and convention rules say. It's not that mysterious a process. No matter what Biden's real or perceived failings as a candidate, he'll be the nominee unless someone that can get more than a bare handful of delegates enters the process.

No one has, and no one will, so Biden will be the nominee.