Why Japan's planned 28 light cruisers would have limited wartime value

How much difference would Japan's full planned force of 28 Sendai-, Nagara-, Yubari-, Kuma-, and Tenryu-class light cruisers make in World War II?

Very little: most were too small and dated for front-line cruiser combat, and Japan was unlikely to convert them into the anti-aircraft or convoy-escort ships that might have made them useful.

The proposed total comprises eight Sendais, six Nagaras, one Yubari, five Kumas, and eight Tenryus. Most were small, older designs vulnerable to opponents with modern fire control. Anti-aircraft conversion with the 100 mm guns used in Japan's dedicated air-defence destroyers could make some hulls useful, provided enough guns were produced and a light cruiser armed with that calibre was institutionally acceptable. Fitting the more familiar 5-inch and 25 mm weapons would offer a weaker result. Torpedo conversions have a limited possible use as oversized destroyer leaders around Guadalcanal. Mogami with its triple 6-inch turrets offers a more capable alternative for fighting modern light cruisers. A later light-cruiser hull could instead sacrifice its extensive after aircraft arrangements for anti-aircraft or anti-submarine equipment and serve as a convoy-escort flagship. These possibilities still depend on industrial capacity and different employment decisions, so completing the original light-cruiser programme would not remedy Japan's larger wartime weaknesses.