You cant build popular support for dismantling the system as it is while you're actively advocating for people to accept the lesser evil.
Imagine if Sanders got up on the senate floor and said "i believe we cannot compromise on ACA subsidies and let millions of americans lose health coverage, be forced to ration their insulin or die because they cant afford a doctor, but I'll be voting to reopen the government without them anyway because i have no choice".
Democrats rely on the inherent violence of a 2 party system. Playing into it isnt pragmatic, it's denial. Either we're in this together or we aren't, and democrats have made it perfectly clear that they aren't.