I think a large organization like Google, with thousands of engineers can safely fire say 10% without loosing much productivity. I refuse to believe that Google has some magic hiring strategy that allows them to hire only smart, motivated and productive people. I'm sure they, like every other large corporation (PIP agreement?!) have a 10% "fat layer" of people who are not very motivated, productive, etc. I'm not saying these people are stupid, they just might not fit in or whatever.