Hmmm…you seem to think Biden’s replacement would be more center/moderate than Biden himself.*
Maybe, but that would be a very unusual take on things. Most would say that most progressives “held their noses” (in the primaries, and certainly in the general) and voted for Biden DESPITE his centrism, because, hey, it’s better than Trump.
Indeed, many in the Dem primaries voted for Biden and not Bernie Sanders (just like they did for Hillary and not Sanders in ‘16), despite preferring many of Sanders’ proposals, because the critical goal was to attract just enough independent, white, blue-collar voters — who would vote for Joe in places like Scranton, Pennsylvania — to block a Trump re-election.
(Perhaps, as a non-US person, you are unaware of Biden’s half-century of centrism. And perhaps you were using his VP pick of Harris as a sign of progressivism — when in fact it was a sign, in part, of his need to attract progressives — not that Harris is all that progressive, really).
(Also, in your country, do party structures “pick” a candidate? Not here, they don’t. The DNC and RNC have some influence at a few moments in the process — e.g., allocating funds for certain local/regional races — but not as much as you seem to think.)
*Personally, I think his replacement, if any, should be just about the same as Biden in terms of centrism/moderation — again, to not turn off those folks in Scranton, PA and the like. I don’t enjoy catering so much to this not-huge group of folks, but for now, it seems we must.