Respectfully, I think you (and some other posters) are missing the point.
The question is not whether you can trust Warren. You can't.
The question is whether we can build enough independent power to make her - or anyone else's - inevitable backpedaling political suicide.
Politics is about power. It isn't about finding honest, morally upstanding leaders to elect. It isn't a spectator sport.
Universal care will not happen unless regular, everyday people - patients and med students and doctors and nurses' unions and small businesspersons - make it happen. That's the fight right now in California, and it will be the fight at the national level, one way or another. The Democratic Party is practically run by lobbyists, the economic and political system is dominated by people who profit from suffering, and neither will give an inch unless they are forced to.
Given that, if Warren wants to talk up single-payer, if she wants to help us paint a bold vision of the future we could have to contrast with the present we are made to endure, I will gladly cheer her on.