Breaking a cryptosystem is not a good metric for whether quantum computers have become useful. Calculating energy levels of molecules, for example, is a much easier problem in the near term, and is of benefit to humanity. Breaking a cryptosystem takes vastly more and higher-quality qubits, and the end result is just that everyone upgrades to different math.

As a computational chemist, I assign 50% probability to the idea that quantum computers will be able to do useful molecular calculations cheaper than classical computers (for large batches) within 18 years. For small batches, classical computers will remain cheapest much longer because of overhead.