I think the whole "lack of a primary" was really a non issue for a number of reasons.
First of all, the only people who think its a problem were MAGAchud who weren't going to vote for the democrats anyways. (I doubt very much whether there were a significant number of voters in 2024 who thought "I really dislike Trump because he is a racist felon, but I'll vote for him anyways because the Democrats didn't have a proper primary".)
Secondly, primaries themselves are a problem. They cost money to run (cash that could be better spent on the general election), and the infighting amongst candidates might leave the eventual winner damaged going into the general election.
Plus, since Biden withdrew rather late into the election cycle, the Democrats would have had to alter the primary process (restart things/condense the process and alter the schedule/etc.) And I am sure that whatever they did, the MAGAchud would still find a reason to complain. "Even though I was always going to vote for a racist con artist, I will pretend like I care about how the Democrats chose their leader and complain because the state primaries were too close together/didn't have enough time for candidates to plan/etc."