The bullshit of the faux libertarians aside, and back to the OP …..
I think Warren really can pay for this without more taxes on the middle class. She can't use her "wealth tax" because she's spent that elsewhere. (sort of like the gop spent the Obamacare money to pay for the first tax cut, so the second one is all borrowed money)
But there's no way she can pay for Medicare for all without clawing back every dime in tax cuts for employers to provide care, and every dime people would have to pay out of pocket for co-pays and uncovered doctor office visits. That is, it'd still have to cost workers as much out of pocket as what they have. And like Obama's fated "you can keep your insurance if you like it" promise, some people would do better and some worse under her plan. Not unless she finds a lot of money somewhere.
And the reality of her situation is that medicare for all is not a winning issue if people can't keep their private HC instead by opting out, and there's no way she can do THAT. She picked this issue to be as left as Bernie and be different from Biden. Ironically Biden would win on the economic issues, but he's old and his kid is not much better than Trump's spawn.
And there's no way for Warren to tack back to the middle now. She'd lose her own base.