This was also a subversion of the will of primary voters. It's not a defense. A huge portion of Democratic voters wanted the candidate replaced, and there were Democratic politicians that announced their intention to be the candidate that replaced him, and others undoubtedly who would have if there hadn't been so much pressure not to, yet a serious primary wasn't run.
Democratic insiders decided that no one was going to run against Biden, Democratic insiders decided after the debate that Biden was not going to run. Democratic insiders will also be choosing who will run. The most democratic and likely option will be Harris (because that's her job) but she was also chosen by Democratic insiders in 2020, not by Democratic voters, who mostly hated her. If not Harris, then Democratic insiders will either choose an arbitrary process or simply directly choose another candidate.