When you remove/nerf mechanics and champions intended to counter something

That certain something will have to suffer subsequent nerfs so that their power is scaled accordingly. For 3 seasons counters to Mages have been nerfed - silences were removed, Assassins were nerfed again and again, Anti-Mages were nerfed again and again, Assassins and all Anti-Mages were reworked, armor penetration was nerfed and the list continues. And now it's coming full circle, the reason why AP itemisation isn't as absurdly strong as it was in season 5 (120 AP Zhonyas? Whew. 100 Ap Rylais? Cheap Rabadons?) is because what was meant to check and balance them has been nerfed, so they have to be scaled down too otherwise they'd be completely and utterly broken.

It's a cycle, if one thing is nerfed then something else needs to be nerfed to maintain the equilibrium, or compensation needs to be given, and it's now AP's turn to be on the receiving end of this cycle. EoN is a pretty good example. The item is retarded, probably shouldn't exist, but was implemented because people cried about Assassin burst, so now Assassins need crappy, gimmicky items like this to deliver their 'sustained bursty dps' garbage instead.