Why a 6-inch gunboat was not a cruiser substitute

Why did navies not exploit treaty-exempt gunboats by fitting them with 6-inch guns?

Because the resulting ships would still have little value in a naval battle; their useful roles were riverine warfare and amphibious support.

A gunboat carrying 6-inch guns still had very little fighting value against other warships. Its useful roles were amphibious support and riverine operations. Those missions justified an exemption because navies needed gunboats, but they also limited how many such vessels were useful. Fitting a substantial gun did not make the ship an effective participant in a fleet battle.