The government has no obligation to continue any program. Although subsidsys have been promised it is not a contract.

Lets take a more extreeme example. Lets say the government throws out social security. Even if you are 64 and have paid into it for 40+ years, you have no standing to sue.

that would be it?

The Supreme Court can overturn any previous Supreme Court decision, but it rarely does.

The Constitution is relatively stable, but it contains the procedure for modification by Amendment, including a constitutional convention, which could rewrite the constitution in its entirety.

That's what congress if for. They can make a law and repeal it whenever they want as long as the sitting pres. signs off on it. If for some reason the ACA would be repealed then like any other repealed law it would simply cease to exist. They can also repeal parts of the law if they want to. If congress decides to repeal just the tax credits (what everyone calls subsidies though they aren't since you have to pay them yourself up front and the government then reimburses you at tax time), and the pres. signed off on it then we'd have all the other parts of the ACE except the tax credits. They can repeal parts, they can add parts, they can do whatever the hell they want to it just as with any other law just as they've always done.

You could still sue since anyone can sue for anything, but you wouldn't win. Congress can make or repeal any law they want, and as long as the sitting pres. signs off on it that's just the way it is and has always been. It's how our government was set up. All the people can do is vote or not vote for any congressperson or president that either makes or repeals a law they either like or dislike according to what they believe should be.

But it isn't so easy to get rid of a law that the people like as in Medicare and Social Security. Congress nor the sitting pres. could ever get away with doing away with either and ever hope to be in politics again which is why congress and presidents chip away at those things that the people like and figure out ways to try fooling the people that they don't have a choice... the whole "Gee, we wish we didn't have to make Medicare/Social Security/whatever worse, but the budget (or this or that, blah, blah, blah) so we have to do it." That's how the government has chipped away at laws the people like and get away with it. .. if they convince enough people that "gee, we don't want to but we have to" that will still vote for them they bloody well WILL do it.

In Flemming v. Nestor, 363 U.S. 603 (1960) the court said Social Security could be changed at any time by Congress. There is no 'right' to specific benefits.