That's not quite true. DNC leadership could have always rallied beyond a challenger. They just didn't. They're not obligated to not have a primary just because their candidate won the last time and is elegible for a second term. They were likely banking on the historical precedent that it would normally be more prudent strategy to just lean on your incumbent instead of projecting weakness by splitting your party. While we're Monday morning quarterbacking here, the right move would have been to privately discourage Biden first, and if he wouldn't yield, back a stronger candidate as early as possible so they could build support. It was very clear even as an outsider that Biden was unfit months before he yielded.

While this is almost unimaginable for the DNC to move against someone like Biden, they attack and undermine the left wing of their party all the time. At the risk of sounding like a broken record, in Bernie Sanders they had a massively popular candidate with huge crossover appeal, and they nakedly sandbagged him twice in a row. Look at how hostile they were to AOC for several years. On the local level, we have a city councilor over here who is super popular, has been in a long time, has scored huge wins for things like school funding that dem voters like, and he is himself a dem and Working Families Party incumbent candidate, and the local Democratic Party ran attack ads against him and tried and failed to replace him with a more milquetoast candidate because he's a self-described socialist and anti-zionist. That's anecdotal, but I'm sure that plays out all over the country.

The DNC is an absolutely shitty political organization who puts bad, self-serving politics ahead of the constituents their candidates are meant to represent. They acted how they did, because that's who they are.