Where does this fail? The transposition of laboratory benchmarks to real market conditions is the weak link here.
A quantum system with 64 qubits, even for 250 assets, is far from the robustness requirements for an active portfolio worth several billion.
Fund managers are subject to constraints of reliability and rebalancing speed that quantum cannot currently meet.
Consider the frequency of critical rebalancing for a hedge fund; classical solutions remain faster and more reliable in the face of operational risks.