I wonder what's fueling all this FUD about Biden not running.

IMHO, mostly it's venting. At least on the Democratic/progressive side. They're nervous because Biden has some obvious flaws, they'd rather go into an election with a candidate with no obvious flaws, and are engaged in a little wishing.

Part of it is jockeying for policies. One way to try to convince an Administration that isn't doing what you want is to try to make them afraid that it will prevent their re-election. So the progressive wing, which is **very** unhappy with some of Biden's choices, are agitating to get him to make different choices.

And part of it is just a clever framing for hitting an opponent with negatives. The GOP is going to be running a lot of "Biden's too old to be President" attack ads in the upcoming campaign. Fueling a discussion of, "Is Biden so old that he should reconsider running?" is an effective additional way of framing that message.