You know what's worse than a Democrat who's against Medicare For All? A Democrat who comes out early as a leading proponent of Medicare For All, only to later completely backtrack by saying they're not in favor of abolishing private health insurance (a critical feature of any Single Payer system). Better to come to battle with an army half as big than for half your army to turn on you mid battle. 

Elizabeth Warren is an ally to the movement only so far as it can benefit her personally. That's not an ally.