Yet still that was never enough for a clear and definitive "no".
It is very likely that the people in favor of this would still try to push it through, or let that happen. They know that the legal battle afterwards to determine its unlawfulness would take years.
And during that time it could already be put it place. And once the legal battle is over (and likely won) severe damage is done and they could still adapt the law or just offer companies to continue doing this "voluntarily". And personally I wouldn't count on Apple, Google, or Facebook to roll this back quickly in that case once they've put it into place.
The most obvious mechanism is a constitutional amendment, but in the U.S. the only amendment to be drafted and adopted in modern times is the 26th amendment (1971), 54 years ago. (The 27th amendment had a weird status where it was belatedly adopted with a 200-year delay.) It's hard to imagine many constitutional amendments actually being passed now because it's been challenging to find consensus on many things within U.S. politics lately.
I don't know that the EU at a supranational level has any mechanism at all to ban future EU directives. Maybe they could decide to remove something from the list of areas of competence of the EU? But Chat Control is under the "Area of Freedom, Security and Justice" and I can't imagine the EU deciding that that should be abandoned as an area of Union competence.
Edit: The international human rights treaties, at least in regulating law enforcement, have tended not to follow the idea that some kind of regulation or law enforcement power is completely off-limits, but just that they need procedural safeguards -- especially for surveillance and investigatory powers. In this case, Chat Control opponents (including me) would like it to be completely off-limits, but the human rights instruments arguments might more naturally go into "did they create enough surrounding rules and mechanisms about how it's used and how it's regulated?" rather than "can we just say governments just can't make this rule?".
It's the task of parliaments, governments, and courts to reevaluate and resolve all these contradictions over and over again. It's tedious and takes a lot of resources, but that's the price for democracy.
The constitution of the Democratic People's Republic of Korea (i.e. North Korea) famously guarantees freedom of expression as a fundamental right for the people. That hasn't stopped the government from trampling all over freedom of expression, though. The EU is of course nowhere near North Korea in terms of what is considered acceptable, but don't ever trust that the words in the constitution will be enough to keep the government from doing something.
