I disagree. I think it has everything to do with hardware because the software has to work with the hardware. The teams more than likely focus on making sure new hardware works with the OS as a priority and focus on the "nice to haves" for point releases. Rosetta, for example, probably took up a large chunk of time in the latest release and that has to extend to nearly every aspect of the UI to make sure that notifications, power management, and resource management all work properly with both new and existing apps on both ARM and x64 versions.