That's because the ACA works through private insurance infrastructure not the federal government.
There are several rulings to pull from including that there is no purview in the Constitution for the federal government to legally provide insurance unless you agree that healthcare falls under the General Welfare clause of the Constitution. And that question will be asked by any judge considered by Republicans 100% because General Welfare clause cover a large swath of entitlements/welfare and is the bane of Republicans existence.
Also, right to privacy will he challenged and there is precedence that the federal government should not have access to our private medical information and therefore is not legally allowed to provide insurance without private options for people who are concerned about the federal government having private information.
The other legal argument against in that same area would be that if federal government can demand private information for healthcare it can for other facets of government as well.
It will be challenged and will be decided by Supreme Court. 100%
The judges appointed by other Democratic presidents in less progressively friendly electorates says otherwise.
Plus Congress has a say, it isn't a unilateral decision. Including those progressives I'm Congress in which he will need their support.
And not sure how going 6-3 in the other direction for next few decades gets us closer. In fact it ends the dream. Even at 5-4 it's already a pipe dream until a conservative judge retires or passes.