Ubuntu's window buttons to stay on the left

Furthermore, if the aim of moving these buttons to the left was to add Exciting New Features^TM on the right, then you also run into the issue (perhaps not immediately, but eventually) that the older themes just simply aren't supported well. It's the familiar pattern of interface deprecation. Mark the now-out-of-vogue way as "deprecated," meaning that you can keep using it for now, but it'll get removed at some point in the future because it's no longer "the way."

You now have partitioned the community into the "well supported" and "legacy users" with respect to this UI element. While it makes sense to deprecate some things, providing backward compatibility "for a time", I'm not sure deprecating such a widely used interface element makes sense. Even Windows 7 has kept the buttons over there (albeit, applying Fitts' Law more effectively than in the past, with the larger and variable-width buttons).