Reserving CPU for itself will make all third party app run slower. And third party apps are a huge selling point (look at Windows Phone), you don't want to be behind your competition because you want a smoother experience on your stock applications.

Also there is feature creep, the longer the system goes on the more feature, security checks, fixed, etc... your basic system does and that will exponentially increase the weigth of your system, requiring you to serve more and more, and making everything else slower.