Quantum theory predicts that they will work given enough time. If they don't work, there is something about physics that we are missing.
The way to test out this theory is to try out an experiment to see if this is so. If this experiment fails, we'll have to figure out why theory predicted it but the experiment didn't deliver.
Quantum theory doesn't predict that it's possible to build a large scale quantum computer. It merely says that a large scale quantum computer is consistent with theory.
Dyson spheres and space elevators are also consistent with quantum theory, but that doesn't mean that it's possible to build one.
Physical theories are subtractive, something that is consistent with the lowest levels of theory can still be ruled out by higher levels.
Quantum theory, like all physical theories, makes predictions. In this case, quantum theory predicts that if the physical error rate of qubits is below a threshold, then error correction can be used to increase the quality of a logical at arbitrarily high levels. This prediction can be false. We currently don't know all of the potential noise sources that will prevent us from building a quantum logic gate that is of similar quality as a classical logic gate.
Building thousands of these logical qubits is an engineering problem similar to Dyson spheres and space elevators. You're right that the lower levels of building 1 really good logical qubit doesn't mean that we can build thousands of them.
If our case, even the lower-levels haven't been validated. This is what I meant when I implied that the project of building a large-scale QC might teach us something new about physics.
If "this experiment" is trying to build a machine, then failure doesn't give much evidence against the theory. Most machine-building failures are caused by insufficient hardware/engineering.
In the specific spot I commented, I guess you were just talking about the physics part? But the GP was talking about both physics and physical realization, so I thought you were also talking about the combination too.
Yes we can probably test the quantum theory. But verifying the physics isn't what this comment chain is really about. It's about working machines. With enough reliable qubits to do useful work.
On the engineering end, the question on if a large-scale quantum computer can be built is leaning to be "yes" so far. DARPA QBI https://www.darpa.mil/research/programs/quantum-benchmarking... was made to answer this question and 11 teams have made it to Stage B. Of course, only people who believe DARPA will trust this evidence, but that's all I have to go on.
On the application front, the jury is still out for applications that are not related to simulation or cryptography: https://arxiv.org/abs/2511.09124