Why KGV's 5.25-inch guns were not simply wasted weight

How much would King George V gain by using 4.5-inch dual-purpose guns instead of 5.25-inch guns?

It might gain some flexibility and weight margin, but not enough for a major redesign; the 5.25-inch gun better fills both surface and anti-aircraft roles.

Switching to 4.5-inch guns leaves limited room for extra mounts or easier logistics, rather than a decisive improvement in armor or main armament. Designers might use the margin for another pair of mounts or a cleaner ammunition system. The 5.25-inch weapon is judged better suited to a capital ship because it combines anti-aircraft work with a useful cruiser-engagement capability. The trade concerns secondary-battery balance rather than unlocking a fundamentally different battleship.