What are the real world use cases now, today? The only thing I see in the QC space, are QC stocks and funding paying for the employment of scientific experimentation, which isn't a real world application.
Do I have to wait 15 to 30 years for a series of real world changing breakthroughs that I can already do on a NVIDIA GPU card?
That doesn't exponential at all, in fact that sounds very very bearish.
Then invest accordingly, and later reinvest your winnings in a different direction.
There are no real world use cases today. The hardware is not advanced enough yet, but it's improving exponentially.
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.