You are right that not all complexity is gone regarding modern FPS. However, the mechanics have dumbed down really hard. Movement got pretty irrelevant with the rise of consoles and devs having to adopt to controllers. In actual fact games such as Quake 3 or Unreal Tournament are impossible to play on a controller.

I guess all kinds of games are some kind of IQ test. In a RPG you might confront the player with several ways how to combine the abilities of your characters efficiently. In a RTS you want to have a smart unit combination etc

I´ve always been interested in the view of devs themselfes. Upvote and follow.