You seem to forget that a new version can introduce new bugs.
Also, if they decided that they were going to support 4.5 on XP, then their ability to detect bugs would then be diluted. So, not only could they introduce new bugs, but they wouldn't be as effective at tracking them down.
I don't think people realize how complicated it is to test even the smallest things. Have a look at the test labs sometime. They have thousands of machines with different configurations running bajillions of tests. It annoys me when I think about even what it takes for somebody in a small shop to successfully roll out a change, and then hear people insist that somebody the size of Microsoft should make a certain change, because it's "a really simple thing to do".