> One of those predictions is that it's possible to build a large-scale fault tolerant quantum computer.
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 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.