What's the "critical mass" needed for the D's to nominate someone other than Biden in 2024?

Yes- because Republicans are liking him. Which means nothing in a Dem primary, and very little in the General. Few Republicans will pick any Dem, even Manchin over trump.

** Over the past year, Manchin has doubled his approval rating among West Virginia Republicans, to 69%.*

Exactly. So a big move to the left means a loss.

Buttigieg is openly gay, and a instant loss, not to mention he has little experience.

I do not care what Republicans think of her. The 538 polls I showed with just Dems, shows her beating everyone but Biden, and usually beating trump, but there it is very close. Popularity today means nothing, since EVERY.SINGLE. REPUBLICAN will rank every single Dem as “utterly craptastic” with the apparent exception of Manchin in his own state.