Why the 4.7-inch dual-purpose mount was not the biggest treaty mistake

Was failing to develop a high-angle dual-purpose mount for the British 4.7-inch quick-firing gun the biggest treaty-era mistake?

No. It would have been useful, while broader treaty-linked force-structure failures rank higher.

A proper high-angle dual-purpose mounting for the British 4.7-inch quick-firing gun would have been useful, especially for destroyers. Its absence ranks below the largest missed opportunities considered here. The question is one of relative importance across the treaty nations. A useful armament improvement does not automatically outweigh a larger change in the composition of a navy.