Why most pre-C-class cruisers became obsolete after 1918

Why did the Royal Navy face apparent block obsolescence across its cruiser fleet after the First World War?

Cruiser doctrine had shifted to faster, more heavily armed ships, making most pre-C-class designs too slow or too lightly armed for modern roles.

The problem was a doctrinal and technical threshold rather than age alone. After the First World War, a useful general-purpose cruiser was expected to make about 28 knots and carry guns of roughly 6-inch caliber. Many older vessels could meet neither requirement, while destroyers increasingly covered tasks once assigned to small cruisers. The C class stood near the new minimum and could be adapted into specialized anti-aircraft ships. The result looked like block obsolescence because a wartime paradigm shift invalidated many designs at the same moment.