This time is needed to 'prime the channel'. The final lock-down is GM, and this usually happens around 1 month before the first customers see the next OS. This is why, in the process of reaching a GM build, the ability for groups to change components is gradually locked down. The more things change, the more you need to test and retest to be absolutely sure that those changes didn't have any unintended side-effects.
So, when you build something as big and complex as an operating system, your single biggest enemy is: change.
Let me tell you a little story from the inside…
