Apple does go through heroic lengths to preserve backward compatibility, but only for as long as it makes sense. If the backwards compatibility is going to harm moving the OS forward, then they are prepared to drop it, as in this case. Microsoft has always been too afraid to drop backwards compatibility (mostly because as soon as they do, many users will probably jump switch to Mac OS if they are forced to upgrade everything anyway), forcing them to remain stuck with legacy Windows code while Apple has continued to push forward. The jump from Mac OS 9 to OS X was a huge one that Microsoft still hasn't been able to take, even with Windows 7.
