Apple was able to do it, but not without both a compatibility layer for direct binary compatibility (Classic) and a migration path to allow the same source code

Apple was able to do it, but not without both a compatibility layer for direct binary compatibility (Classic) and a migration path to allow the same source code to be compiled for both OS 9 and OS X (Carbon). Not to mention the heavy Java push they made for a while, which didn't end up becoming much in the long run but did provide one more way for developers to get their applications onto the platform. And developers like Omni that followed NEXT after their purchase by Apple.