Having a unit type in your toolkit is one thing, and expecting it to play a decisive role in battle is another. For example, Alexander's heavy infantry was supposed to anchor the enemy in place as cavalry won the battle, while Roman heavy infantry the primary fighting force.

Mercenary pikemen ended the dominance of heavy cavalry in Europe before firearms became common. But that didn't make heavy cavalry obsolete, and neither did firearms. Cuirassiers had a prominent role in the Napoleonic Wars, and French cuirassiers actually wore their breastplates a few weeks into WW1.