Why not answer 16.5-inch guns with a longer 16-inch weapon?

Could the United States have answered British 16.5-inch guns with a longer-ranged 16-inch/50 weapon instead of moving to 18 inches?

Not easily in the 1910s. Manufacturing a much longer barrel was a harder metallurgical problem than increasing caliber with the methods then available.

Barrel length was not a free route to velocity and range. A longer gun placed demanding stresses on the metallurgy and construction techniques used to make the tube. Britain faced the same constraint when it considered longer 15-inch variants and settled on the workable 15-inch/42. An apparently modest change in caliber could be more practical than a high-performance long-barreled weapon.