Of course, Apple doesn't make it easy to upgrade to their stable and regressed operating system - I'm going to have go through all sorts of hoops to upgrade to anything other than the (still not fully trusted by me) Yosemite.
However, if one is using Xcode to write iOS 8 apps for the millions of new phones people bought a few months ago, you are pretty much forced onto the Apple upgrade treadmill.
For example, Xcode 6.3 requires 10.10[1]. Apple decided that OS X 10.9 (Mavericks) released less than 18 months ago is not recent enough.
Staying with OS X 10.6 means the latest Xcode you can install is v4 which means iOS 5 is the last version you can write apps for. That would be a severe handicap since iOS 5 has less than 3% marketshare[2]. I'm not sure if Apple servers would even allow developers to submit an iOS 5 app these days[3]. OS X 10.6 definitely was very stable but staying on it has severe economic penalties for many programmers.
[1]https://itunes.apple.com/us/app/xcode/id497799835?ls=1&mt=12
[2]http://david-smith.org/iosversionstats/
[3]http://www.macworld.com/article/2080865/apple-requiring-app-...
Snow Leopard is still my favorite. Mountain Lion was ok and Mavericks and Yosemite have been error-filled and support headaches. I am hoping the next version is better tested.
I agree about the testing, but still I think they have managed to keep the technical debt somewhat in control. I really yearn for the day they license Plan9, and port OS X on top of that. Because I believe that would give bigger opportunities, both with respect to hardware, but also with respect to a better foundation for developing more sophisticated features.
[edit: I should say that OpenStep was my favorite release and somethings are still not in OS X which is a pity - as to Plan 9 - I do have dreams of OpenStep on top of Plan 9]