Are there any good reasons why the navies of the world kept their protected cruisers in service? Astraea-class, one of my favorites, was in service for twenty plus years with the Royal Navy. Germany and the United States still had protected cruisers as well with the USS Olympia C6 as one of the oldest steel vessels still afloat.
These protected cruisers often had name changes to second-class cruisers and the such, but their faults still remained. With weak armor and limit use these vessels were best for showing the flag in the colonies. And it was in the colonies that these vessels were able to assist in the war effort. Protecting trade and landing shore parties were needed.
The reason for my interest in Astraea-class is that one of these cruisers, HMS Fox, was to be part of the invasion of Tanga in German East Africa. Unfortunately her guns were never put to use against the Germans in Tanga. They were used successfully along the Red Sea supporting the Arab Revolt.
The USS Olympia was used on the fringes of the war when she was part of the American intervention in and around Murmansk.
With better armor and armor protection and more efficient engines the protected cruisers and armored cruisers were coming to an end replaced in service by light cruisers and battle cruisers.
The lack of protection and poor training helped bring it all to an end. On the 22nd of September 1914. U-9 and six torpedoes sunk three of the Cressey-class. It shook up the Admiralty and ships of limited use were regulated to the farthest corners of the empire.