If you want to nerf hh/berserkers, riflemen, maybe fiends, why not nerf medium armor type. These units are not just strong because of piercing damage, but also because of the medium armor type, which ist the best ‘normal units’ armor type (excluding fortified).
The nerf to piercing also affects hippogryph riders, faerie dragons, wind riders, dragonhawk riders. Are these units supposed to be nerfed? I don’t think so. They aren’t dominating the meta.
The medium armor type is not shared by many other fighting units, mostly only workers and a few others, which could be addressed otherwise.
Imo the medium armor type is too strong against siege (units). It could also be nerfed regarding piercing or magic attacks to make other army compositions viable.
They deal less damage to heavy armor, but what heavy armor units can even attack these units? I can only think of water elemental.
Part of what makes this complicated is that, as you are aware, Medium armor is also the armor type for Workers. I do think the overall strength of Medium armor against Siege attack is weird, but I’d be afraid of messing with the harassability of workers without a looot of thought put into it.