Why a super-battleship still benefits from 6-inch secondaries

Would a battleship with 20-inch-plus main guns need 8- to 10-inch secondary guns?

No. Six-inch weapons offer the rate of fire needed against destroyers and cruisers, while larger targets warrant the main battery.

Fast-firing 6-inch guns remain effective against destroyers and cruisers even when the battleship's main battery exceeds 20 inches. Any target too heavily protected for that secondary battery is important enough to justify main-battery fire. Adding an 8- or 10-inch intermediate battery creates another ammunition and fire-control system while reducing the number of quicker weapons available. German or Japanese designers might still prefer multiple specialized calibers, but the 6-inch choice is more efficient. Secondary armament should match the targets it will actually fight.