Why battlecruisers were not simply second-class battleships

Were battlecruisers merely second-class battleships after 1930?

No. Their speed and strategic mobility still had a distinct purpose, but improving machinery made fast battleships capable enough that the battlecruiser's extra speed became harder to justify.

Gun calibre alone did not make battlecruisers second-class battleships: the Kongos carried 14-inch guns, as did the later King George V battleships. The personal gun preference here is 16.5 inches, with a 16-inch/50-calibre weapon accepted as a practical alternative balancing weight, firing rate, and hitting power. Battlecruisers also retained distinct work with carriers, in trade warfare, and in moving heavy firepower over long distances. Their advantage weakened as higher-pressure boilers and more efficient engines let battleships combine substantial armour with higher speed. A 27-knot ship had a large advantage over a battle line moving at 18 or 21 knots. An extra three to eight knots mattered less once battleships themselves could make roughly 28 to 30 knots. The remaining gain had to justify the protection and other capabilities sacrificed to obtain it. Cruising speed and the ability to reach a distant crisis therefore mattered alongside maximum speed.