Dems held no primary, Biden strong armed his way into the de-facto nomination for a second term, and was replaced by his VP when he imploded on the campaign trail.

Wrong., Dems held their primaries as usual, and Biden won them as the incumbent (almost) always does. https://en.wikipedia.org/wiki/Results_of_the_2024_Democratic_Party_presidential_primaries

Of course he shouldn't have run again, but he did. Maybe some other Dems should have challenged him in the primaries, but they didn't, and they would have failed anyway. By the time he dropped out there was no time to hold another primary and still get a campaign going for a new nominee. So pretty much all they could do was rally around the VP as the successor.