Why navies could not replace battleships with cruiser swarms

Why did treaty-era navies not abandon the battleship race and build large numbers of smaller 9- to 12-inch-gun cruisers for mission kills?

Because the apparent numerical advantage shrank under treaty limits, logistics, and crew requirements, while heavy ships offered greater range and a more reliable answer to an enemy capital ship when a smaller vessel's mission-kill attempt failed.

Smaller warships could swarm a larger opponent or disable exposed systems, but that approach made national security depend on several uncertain events going right. The Royal Navy's cruisers helped mission-kill Scharnhorst at North Cape, yet Duke of York still supplied the decisive heavy-gun force; at the River Plate, Graf Spee's 11-inch guns gave her a theoretical overmatch that British cruiser tactics had to overcome. Treaty accounting also undercut the numerical bargain because an 18,000-ton cruiser would consume capital-ship tonnage and yield closer to two ships for each battleship than the imagined three or four. Alaska required nearly battleship-scale crewing and support while remaining inferior to Iowa in a one-for-one fight.