Unprotected cruisers and the survivability problem

Did gun cruisers take a wrong turn, and what did navies get wrong?

Yes. Unprotected cruisers sacrificed the minimum armor needed for wartime survivability, showing that a little protection could matter far more than its weight suggested.

Unprotected cruisers separate parade or presence value from fighting value. They can look attractive for a while, especially for a navy trying to claim more hulls, but a cruiser needs some survivability because it inherits the frigate's independent wartime role. Even limited protection over engines and magazines mattered in the transition from presence ship to credible cruiser.