When faced with a candidate who will move things forward less quickly than you'd like and another who will take giant steps backward in every respect... well, it shouldn't be that hard a decision.
Yeah, Sanders theory died on 2020 when they didn't show in the primary.
Contrary to what leftists believe, most Americans are fine with incremental progress. Most Dems would prefer it to be faster, but when the GOP is a baked in opposition, you move as far forward as you can with the votes you got.