Call me a schizo, but I think NijiEN caused an EN chuuba burnout that affected both Irys and the Council. If there was just a solid year of no big EN chuubas between Myth and Council, people would've attached to the new girls immediately, seeing them as the breath of fresh air they finally needed.
Instead, NijiEN debuted, and a lot of Holofags went, "oh nice, a new HoloEN gen!", and by the time the actual new HoloEN gen debuted, fans were like, "ehh, I'll check out the debut, but between Ame, Elira, Gura, and Selen, I just don't have the time, so maybe I'll come back later". So sure, Nijis didn't kill or even match HoloEN, but you'd be crazy to say they haven't affected them at all.