Positioning, prediction, economy, teamwork, movement? Basically - ask any hardcore FPS player what they do besides click on heads. (And then watch them twist in pretzels to insist that clicking on heads is the heart and soul of the game and there'd be nothing left if that was changed in any way.)

Wallhacks can stay forbidden. They're detectable through gameplay. Especially when the server can straight-up lie to players about enemies just around a corner or off in the distance. Dummies can even be sent to the renderer, if they're all masked by cheap occlusion queries. The client does not need to know until a player is nearly onscreen.