Petty leftists weren’t even a significant part of the problem, IMO. Biden is very unpopular, people didn’t want more of the same, and Biden’s vice president looked like more of the same. However, the Democratic party was too hierarchical to nominate the sort of candidate that they needed to nominate.
Hell, they nominated Biden himself even though his age could have given them a perfect excuse not to nominate a sitting president. He was only forced to step aside once his inadequacies were undeniably obvious to all, and even then he was like a child throwing a tantrum. History is going to remember him as the emperor with no clothes.
Many senior Democrats were calling on Biden to resign long before he got covid, but he repeatedly made defiant announcements that he would never resign (and Harris supported him). He’s the guy who said that he would only resign if God told him to! His covid infection appears to have been mild (lasting less than a week), and he resigned not because of it but because pretty much the entire Democratic establishment (led by Pelosi) told him that he must.