* Google has a large and very competent design team. They are not changing things "for the sake of changing them." More often than not, changes are made in response to issues discovered during usability studies.
* You are not the user. I am not the user. The designer is not the user. In fact, for a project as large as Android or Gmail, there is no "the user". You're always making trade-offs between power vs. common, new vs. existing users.
* Existing users are inherently conservative. By definition they mostly like things as they are or they wouldn't be users. You have to weigh their needs vs. those of future users. A big part of that is understanding how many people are upset about the change (all vs. vocal minority), how quickly they get used to it (if ever), and how you can mitigate this.
* Ideally, you want to make "Pareto efficient" changes, that make everyone better off. The only thing easier than doing what you've always done is when the new UX is so easy and intuitive you instantly understand it.
* You do not want to version your UI, just like you don't want to make everything a setting. This is a recipe for maintenance headaches, and endless wasted cycles supporting an ever-shrinking minority of users.
* This being said, it's very helpful to release a redesign as a beta, and let users switch back and forth for a while while you iron out the kinks. This in itself gives you good data.
At the end of the day, design is all about trade-offs. It's very easy to criticize a change because you don't like it, but it's a lot more interesting to think through why a change might have been made. If we're going to be critical, let us be constructive.