My understanding was that quantum volume was an attempt to unify the previously used metric of number of qubits with some concept of potentially being able to do useful work. The last part being why these metrics are elusive. Adding more Transistors to a processor is only useful if we agree they are all beinh used. Same with qubits, in particular if you can't error correct your qubits you can't be confident in the result. So improvement in quantum volume seems to give a better yardstick for the roadmap on general quantum computing.