My intuition is that large scale quantum computing is also in violation of laws of physics, but we don't have good models/theory about those yet.
What I am calling for is a reality check on this field.
Pretty much like string theory I am ready to bet it won't go anywhere.
That possibility alone is worth all of the investment in this field.
In contrast, String Theory is just an extension of QM, motivated by nothing except some cool looking math. It doesn't even make any definite predictions (supersimmetry is possible in string theory, but not required, at least not at any particular energy level).
It is already known that many of the most physically important quantum field theories are actually string theories if you rewrite them in different variables (this is called AdS/CFT duality), especially for understanding their large-coupling behavior string theory is the only tool available to analytically understand the theory. So string theory being completely wrong would be as surprising as quantum computing being impossible, for the same reason as you gave for QC. It would indicate something profoundly wrong about something we think we understand well. Its not impossible but the case is much, much more robust than the general internet understands.
And yes, to the extent that it's the same formulae as QM, finding counter-example would be fascinating. But (higher-dimensional) string theory could be wrong with no impact on QM/QFT.
In other words I’m not saying that string theory is (only) a generalization of known correct physics, but that many string theories are equal to (and therefore by definition as physically valid as) physically important quantum field theories. Then there are other string theories that are truly interesting and novel, too.
Additionally, this conjecture is actually experimentally verifiable—you can set up strongly coupled systems in the lab and study the string theory description, and compare the predictions.