NBA superstars don't accept loosing their skills gracefully. Ray can still shoot, but he finally became too slow, injury prone, predictable to handle a big role. Doc made the right choice.
And as classy as Ray is, he didn't say the right things on the way out the door. He couldn't. A superstar can't let go of what they once were. It goes against their DNA. He was in a no win situation - backed into a corner by his own ego.
What he did for Boston in 2007-2012 was awesome. But going forward the team is going to be better with Rondo remaining in control, and sharing the back court with Bradley, Terry, Lee, as well as a little Pierce and Green.