Why Yubari's test-bed design stayed conservative

Was Yubari too small to serve as both a useful cruiser and an experimental ship?

Her size was workable, but the layout is judged too conservative to make the most of her experimental role.

Yubari was large enough to be a viable cruiser and test ship, but the criticism here is that her experimental layout remained too conservative. Three twin 5.5-inch turrets could have replaced the two twin and two single mounts, probably saving weight and clearing the after main deck for mines and torpedoes. At just under 35 knots, such a ship could serve as a long-range fleet destroyer. Even modest armour would give it an advantage against ordinary destroyers. Building groups of these ships after the Washington treaty and before the London treaty is proposed as a way to gain useful numbers and influence later negotiations. This remained an alternative course Japan did not take.