When you can sell your AAPL stock for bank and the project you've been working on for 7 years is on its 5th iteration, there's little motivation to stick around. And you don't have Steve Jobs around anymore to send defensive emails to people poaching Apple's talent.
Ultimately, the retention of talent will be Apple’s Achilles’ heel.
The smartest people will always want to be working on the smartest thing. Sometimes that comes together in one amazing project. iOS has been that project for this decade.