Nah, you get lots of dead wood at any big company. Nature of the beast. The hiring practices are more like an initiation rite than a real quality indicator.
Plus, 15% would be a pretty small amount of deadwood at a company that size. It may just be specialists hired for departments that don't exist anymore or people that stopped caring 5 years ago, but have been around so long that they can't be fired.
From what I've heard from friends at MS, it seems their dead wood is in management - there's a culture of ass-covering and doing the bare minimum on many teams, which goes a long way to explain why MS rarely does anything revolutionary. Great ideas that are risky don't make it very far in that company.
Exactly 10%-20% isn't that much. An company should be able to fire the worst at the bottom, hire new people to take their place distributed evenly across the spectrum & come out net ahead.
Plus, 15% would be a pretty small amount of deadwood at a company that size. It may just be specialists hired for departments that don't exist anymore or people that stopped caring 5 years ago, but have been around so long that they can't be fired.