Armor thickness alone does not define a ship's type. Coverage and internal subdivision determine how much of the ship can survive a hit, placing armored cruisers, battlecruisers, fast battleships, and dreadnought battleships along a spectrum. Early battlecruisers could replace armored cruisers at tolerable cost. Later ships such as HMS Tiger were too valuable and too few to serve routinely as station flagships or convoy escorts, leaving room for a cheaper 9.2-inch cruiser.
Chapter · 5:03-15:35
Battlecruiser evolution made numbers unaffordable
The progression from armored cruisers through battlecruisers to fast battleships increased armor coverage, subdivision, capability, and cost.