No real competitive Dem primary in 2016 - fixed by Hillary and her henchman, Debbie Wasserman Schultz.
Dems called off primary competition in 2020 to coalesce around Biden after South Carolina.
Biden was supposed to be a single-term placeholder, but, Biden being Biden, he ran for a second term. This meant there would be no competitive 2024 Dem Primary. George Clooney stated the obvious, and Nancy Pelosi had to appoint Kamala as the nominee.
The last competitive Dem Primary was 2008, when an upstart Obama dethroned Her Inevitable Highness, Queen Hillary. Hillary made damn sure that wouldn't happen again when she ran for President in 2016, thus, the 2016 Dem Primary fix was in.
The rise of the DSA is akin to the rise of the Tea Party. The political estbablishments kept a lid on internal competition that alienated upcoming younger members who responded by mounting internal insurgencies against Party Establishment.
Biden's policies made things worse for two Dem constiuencies - college students/recent graduates and working-class minorities. The limousine liberal crowd loved the offshore windfarms, until one befouled beaches in Nantucket.
Hillary Clinton's propensity to insult large Dem constituencies (deplorables, young ignorants) didn't help.