Another way to look at it is that Android was a home run and basically accomplished everything it was supposed to do, not much left for Rubin to accomplish.
Sure, it's probably achieved everything it was meant to - like Google probably had when they first became the top search engine. I really doubt anybody is leaving because of that though, it's certainly not perfect, and it's certainly not achieved everything it can achieve. In fact, it probably hasn't achieved everything it is meant to achieve now. Goals aren't static, nobody's going to leave because there's nothing left to achieve with Android.
