The elephant in the room in an open primary is, of course, the risk of alienating black voters by casting her aside. It's one thing letting her bomb out in 2019 as a lightweight first term Senator from California, when the modus operandi was 'whoever with the most chance of beating Trump', abandoning the black woman VP in favour of most likely a white man would be a fraught decision for the party.
That's why, I suspect, we didn't see any prominent Dem publicly challenge Biden or even so much as floating the idea, best case scenario he wins, worst case scenario they position themselves for a 28 run, and that's also why they quickly coalesced behind Harris, because frankly running as the Democratic candidate when the incumbent Democratic president has 55% unfavourable approval rating is a poisoned chalice.