What caused France's surviving armored cruisers to leave service?
Short answerThe naval treaties.
NotesThe naval treaties ended the surviving French armored cruisers. Continued service was plausible only under a hypothetical Washington Treaty allowance. Their disappearance therefore does not prove that nobody saw any secondary utility in the old ships. Retained hulls might have complicated Italian planning without changing the outcome of the Second World War, but the actual treaty framework removed that option.