Ultraleftists bagging on Bernie are silly. They say he’s sheepdogging, but they’re also promoting their own sects.
I don’t think it’s either-or about supporting Democrats. It’s a big tent party, and the main thing they want is your vote. It’s not like there’s a viable national or even state-level alternative to them (excepting the GOP, but they’re in fashyland).
We may see one or both parties collapse as the status of the American sphere of influence crumbles, but until that happens, we have what we have.
However, we should at least prepare for a collapse of either party, and have infrastructure set up to grow our existing issue-based movements, identity politics groups, and labor organizations.
Maybe study the collapse of the Whigs and the creation of the Republican Party in 1860.
Until then, we have the Democrats, and we have Bernie Sanders at the lead of a progressive, but growing, minority faction within the Democrats.