Let's assume a roughly Earth-like, low-magic setting in which the major civilizations are at the technological level of the European Renaissance, the Ming Dynasty, and/or the later Arab caliphates.

The foundation of this world's main magic system is sympathetic magic comparable to that of Rothfuss' Kingkiller Chronicle series. The main implication for battle magic is that mages can sympathetically connect two objects or substances and affect one by acting on the other. For example, if a mage encounters soldiers with arquebuses or other gunpowder weapons, the mage could sympathetically connect a handful of gunpowder (or a similar volatile substance) to the soldiers' gunpowder reserves and then explode the soldiers' gunpowder by lighting that other volatile substance on fire.

What I've figured out so far, then, is that gunpowder use in combat situations is even more dangerous in my setting than in the real world. On the other hand, my setting's magic doesn't have many applications for battle magic aside from demoralizing enemy troops and performing crude repairs and healing; mages can't cast fireballs and lightning bolts and such. I think, then, my world's armies will end up relying on conventional plate armor, melee weapons, and crossbows for perhaps centuries longer than normal.

What do you think? If gunpowder weapons become non-viable in a low-magic world, what's the trajectory of the history of warfare? What new methods might people develop to kill each other and protect themselves? Would crossbows essentially replace guns for good? Would plate-armored knights still feature on the battlefield into the equivalent of the 19th century?

Edit: For anyone who's unaware, cannons and some firearms were contemporary with plate armor for a couple of centuries in our world.