If they won't deliver on this issue then exactly what will Democrats accomplish when they return to power? Deregulate a few more industries? Tax credits for tech monopolies?
Immigration reform, minimum wage increase, a health care bill that is better than ObamaCare but isn't single payer, tax reform that doesn't involve tax cuts for the top 1%, NAFTA repeal, the Equality Act, etc.
Acting as if single payer is the only worthwhile thing a democratic administration can do for liberal Americans is pretty ridiculous.
It's the most far-reaching, long-term and immediate policy they could do at this point in time. If they can't do a big change, I have no faith in them doing milder reforms.