The design question is what a battlecruiser can realistically be armoured against as opposing main batteries grow. Protection that may offer a meaningful answer to 13.5-inch fire cannot simply be thickened until it reliably defeats 15-, 18-, or speculative 20-inch shells at likely engagement ranges. Some protection can still matter at very long range or against smaller weapons, so armour retains a useful role. Designers must select a realistic threat set instead of spending displacement on an impossible promise of immunity from every capital-ship gun.
Notable Point · 35:27-37:17
Battlecruiser armour cannot answer every heavier gun
As capital-ship guns grow from 13.5 inches toward 15, 18, or speculative 20 inches, practical battlecruiser armour cannot simply scale up to defeat every shell.