Why a battleship refit would skip a small calibre increase

Why would the Royal Navy not replace a 15-inch main battery with 16-inch guns?

The improvement was too small for the cost of rebuilding an existing ship, so a worthwhile replacement would require a much larger step in capability.

The answer distinguishes choosing a gun for a new design from replacing the main battery of a completed ship. A one-inch increase over an established 15-inch weapon would not repay the structural cost of conversion. The discussion therefore expects a more decisive jump, pointing toward experimental 18-inch weapons and the N3 concept rather than a marginal refit. The G3's 16-inch battery is treated separately because its designers were balancing heavy armament against battlecruiser speed in a new hull.