Most of us on this site believe that quantum computing will work. However, let's play devil's advocate. Imagine that we suddenly hit some fundamental stumbling block that prevented further development towards a universal quantum computer. Perhaps we're limited to a NISQ device (Noisy, Intermediate Scale Quantum) of 50-200 qubits, for the sake of argument. The study of (experimental) quantum computing suddenly stops and no further progress is made.
What good has already come out of the study of quantum computers?
By this, I mean realisable quantum technologies, the most obvious candidate being Quantum Key Distribution, but also technical results that feed into other fields. Rather than simply a list of items, a brief description of each would be appreciated.