Several points, but let's start with your assumptions, which you state as fact, but are not.
You're clearly taking the position that if you're not for M4A, then you're "supporting the establishment." This is a classic intellectual fallacy called a "false dichotomy". But worse, it really doesn't address the concerns of the average American.
First, ANY government involved scheme which works to control costs is working "against the establishment". The "establishment" wants to keep the viscious cycle of raising costs paid for by rising premiums going. Any system which throttles the speed at which that cycle moves is "anti-establishment" whether or not its the ACA, a hybrid model like the Dutch and Swiss use, private single payer like the Canadians or Swedes, or public single payer like the Brits.
Thus, you can be both "anti establishment" and "anti-M4A". So let's get that out on the table right now.
Now, on to the salient points:
1) The problem the average american wants solved is the high cost of health care. They couldn't care less about the ideological underpinings of the methodology.
2) The politically ideologic want the high cost problem to be solved in a specific way. The average american...not so much.
3) Any plan that lowers the cost of health care to the average american is going to score high on surveys. There is no survey that proves that the public "wants M4A". The public will vote up ANY plan which lowers their costs. A free (or cheap) lunch is always popular.
4) If you address a political problem to the satisfaction of SOME of the parties that want it solved, part of the support for the more broad solutions starts to peel away.
5) Biden's already stated he wants to expand the ACA. If he infuses more funds into the ACA and finds a way to get all states on board (which is just a matter of money), then the ACA becomes even more popular (and support for replacing the ACA decreases.)
Just that leads me (and should lead you too, also) to conclude that the M4A idea is deader than a doornail.
But, there's more:
6) The M4A advocates know that they can't get the House to approve an M4A bill, so they have this idea that if they can force a vote, this will cause more progressives to be elected in 2022. This is quite naive, because of points 4 and 5 above.
But, have at it. The M4A advocates have centered on a few polls that show broad support for the idea (see 3 above) and decided that they can create a broad coalition around this issue.
They couldn't be more wrong, IMO.