The real issue is that many Americans do not trust the government to keep its promises. If people truly believe that the total costs under M4A will be less than they are now, they'd go for it, even with a tax increase. But the concern that many people have is that if we go all in on M4A, what if they get some huge tax increase and their overall costs actually go up, and there are no private insurer competitors to switch to, and all they get from the government is "sorry, we didn't think it would cost this much, but this is how things are now." You can explain the data and numbers all you want, but unless they trust you, it won't sell.
There are two ways to deal with this. The first is convincing people to just trust you. This is what Bernie and Warren are attempting to do. The second is to push a plan that the data says is shittier and more expensive, but which leaves outs to make people comfortable that they won't be stuck in a situation they hate. This is what Pete and a few others are doing.
I honestly don't know the best option. But since this isn't a health care referendum, I don't think it matters all that much right now, for reasons I've explained before. Plenty of people who are scared of M4A will vote for Warren because they know that there will be a lot of negotiation ahead.