Diana? Yi? lol.... what?
There are a few champions that are brokenly overpowered, vastly outstripping the alternatives in terms of success vs. risk/effort put in. Vayne, Yasuo, Kalista, Lee, Riven, Darius, Gangplank, Mordekaiser, etc. There is never a point where picking these characters would be disadvantageous... they are unequivocally superior to the alternatives barring extremely specific scenarios. (A case can also be made for Elise, Lulu, Gnar and Thresh but not as much)
Riot loves these characters so unlike your Sion, Evelynn, Cassiopeia, Sejuani , etc... they will not get nerfs. Maybe some small love taps , but never a real nerf. Could you ever see a Worlds team select : Cass mid, Rammus jungle, Sion top, Taric and Varus bot lane ? no, you would never see that team in a million years (with this current patch)