fatguyinalittlecoat
Footballguy
Biden's biggest backers appear to be black people and older people. I'm not so sure either of those groups is particularly drawn to Sanders or Warren (admittedly Sanders has done a pretty decent job improving his support in the black community). I think if Biden collapsed at least one of the four candidates you mention would likely grab enough of his voters to get into the conversation as a plausible alternative to Bernia and Warren."Very Unlikely" is the last phrase I would use to characterize the probability of a Biden collapse.
As to your second paragraph you are correct that progressives are not a majority in the Democratic Party. But, assuming for a moment the Biden collapse does happen, let's think about where the support goes. If there was a clear front runner in the Harris/Pete/Klobuchar/Booker clump I'd be more inclined to agree with you. But who do people flock to? Or do they look at Warren/Sanders and default to the next front runner even if they aren't all-in on a progressive platform?