Sure, at a given point with low attrition this may be true. However if you are forcing the "bottom" 10-15% out every year, you tend to run out of "bottom" people to exit.. unless you are very bad at recruiting (or purposely hiring to fire).

Further, in any big organization the weak performers are not evenly distributed. However the PIP / attrition % rates are uniformly forced. So the over performing team has to start cutting into good performers within a year or two, whereas the low performance team can cut and still have many lower performers..