Why the Royal Navy stayed with 4.5-inch guns

Why did the Royal Navy not retain the redesigned high-elevation 4.7-inch mounting for later destroyers?

It chose to standardize on the 4.5-inch system because that offered the better return on continued investment.

The proposal would have reused the M-class weapon lineage in later Battle- or County-type mountings. The Royal Navy preferred to continue with the 4.5-inch system, which it judged to offer a better return on further investment than reviving the alternative 4.7-inch path. The exchange shows how standardization and expected fleet value can outweigh an apparently attractive technical carryover.