Were the final Minotaur armored cruisers and Topaze protected cruisers displaced simply because their designs were outdated?
Short answerThe explanation follows the armored cruiser into the battlecruiser, whose rising size and cost then leave some duties for heavy cruisers. It does not separately trace the protected-cruiser transition.
NotesThe explanation follows the armored cruiser into the battlecruiser as technology and requirements change. Early Invincibles are described as costing relatively little more than the armored cruisers they superseded. By the time of Tiger, battlecruisers have become large and expensive enough to leave room for another generation of heavy cruisers. Those cruisers inherit some of the older armored-cruiser tasks, while the battlecruisers retain others. The question also names protected cruisers, but their design lineage receives no separate explanation here.