Sure, if U operates on both |u> and |v>, their inner product doesn't change. But why does this mean that "nothing ever changes?" |u> can evolve independently of |v>, after all.
OTOH, I can understand the argument that nothing ever happens. One reasonable requirement for something to happen is that it should not be able to "unhappen," which is why some are looking for the possibility of irreversible decoherence.
Eh? If U is a unitary operator that can act on |u>, then U @ I is a unitary operator on the state |u> @ |v>, producing U|u> @ |v>. Can you explain what you mean?
OTOH, I can understand the argument that nothing ever happens. One reasonable requirement for something to happen is that it should not be able to "unhappen," which is why some are looking for the possibility of irreversible decoherence.