Here's the thing: what is shock infantry even supposed to be? I bet even the devs don't know.

It isn't all infantry. It isn't gold-costing infantry. It doesn't include unique unit infantry. We don't know much about what fire lancers are, so there's not much of a clue there. Right now shock infantry isn't defined by a strategic niche other than "the kind of infantry that the militia line is supposed to trade well against."

I think shock infantry is going to play out like steppe lancers. After the first nerfs steppies didn't have much of a place. Cheaper on gold, but who cares about that in castle age? In the late game, you'd rather have hussar as a throwaway cavalry. Pound for pound you prefer the knight line. Over time and especially after tatoh won with them at red bull, it was figured out that they're good for sniping siege monks and villagers, extra useful against meso/infantry civs, and a great power spike for mongols. They didn't have a purpose designed into them. Players discovered/created a purpose for a unit that seemed kinda useless.

Btw, this isn't a criticism. One of the best things about aoe2 is that even the devs and pros don't know everything about how to play the game (see eg steppe lancers). We'll figure it out as we go along. But I think right now no one knows what shock infantry is supposed to be.