You are flat out wrong. First of all, Diana is strong, sure. But she is not the "most OP" champion ever. OP depends on who YOU are playing. For example, Darius is OP when you are playing as Irelia. I'm sorry, but you are screwed if you have to top vs him. As Shen, I kite, build armor, poke and kick him out of lane. Darius players would call Shen OP if they had to fight him.
Secondly, Riot doesn't know how the thousands of players are going to play vs a Diana, or a Darius, or a Nautilus. They can't simulate you and I playing these champions or fighting them. The nerfs are to keep what they thought were balanced and what actually is balanced alined. I see no wicked intention in what they are doing, but if you really do think so then uninstall.
Thirdly, you defeated your own argument by mentioning that Viktor was bad and got nerfed more. You destroyed your grounds for your argument, and Hecarium did not get any nerfs, nor did Varus if I remember correctly. Come up with some better examples before you start a debate.