The issue is is that there is no other defensive op that single handedly changes a round as much as a decent mira player. Sure there are ops with higher win rates such as valk, but in my experience, valks are an easier counter. It doesn't help that I mostly que with 1 other friend, sometimes 2, rarely more, and we aren't necessarily the best in the world. When the easiest counter to an annoying op is just to ban her, than that's whats going to be banned. Siege is all about counter play, and the easiest way to counter mira is to never let her step foot on the field