You don't hold a "proper primary" when you have the advantage of incumbency. You run the incumbent (especially against a candidate they've already beaten) or the VP - the two most famous people in the party. All Harris had to do was read the room and she would have won.

Literally every other possible option would have been a nobody with none of the advantages Biden or Harris had and would have only risked splitting the ticket, whereas every Republican was already going vote for Trump. I can't think of a worse way for the Democrats to fail than that... except for the way they actually failed.

And I mean Trump's physical and mental decline is far worse than Biden's ever was and no one seems to care.