Apple doesn’t craft it’s OS based on how well it runs someone else’s operating system. Intel is way better than PPC, but ARM is way better than Intel. The fact that Macs can run Windows somewhat painlessly is just a coincidence that Apple decided to throw in. If Parallels takes a performance hit because of a switch, that’s not something their going to lose sleep over.
Apple has been making mobile devices with the capacity to run the full OS X for years now, they just haven’t tried to market them as such. They likely have prototypes of every Mac they make running a customized OS X on their A series chips just sitting in a dark room somewhere in Cupertino. And just like the switch to Intel, this is something they would spend a LOT of time perfecting and make dozens of revisions and tweaks to before even thinking about announcing it.
I’ve been waiting for Apple to give their lower-end machines an ARM option as an opening volley for eventually making the processors on everything they sell (Xeon-running Mac Pros probably being the final hold out until there can be some kind of super A chip that could drive an aircraft carrier).
I think that in less than 10 years, Macs will be built pretty much like iOS devices. A single board with processor, memory, and storage built-in (For the 5% that can’t make due with 128/256/512GB of storage, there will always be Thunderbolt drives). The hardware will be advanced enough and the software will be kept as optimized as possible to guarantee at least five years of full support for every product. Software will unify and there will cease to be separate OSes for desktop and mobile. The future Apple OS will behave like a desktop OS on Macs, like a mobile OS on portable devices, like whatever works best for the device it’s on. Software developers will be making one universal build for each app that runs on everything from a wrist band to a server tower, changing its appearance and behavior to suit the device.
That’s just an armchair observation of how things are changing and moving. Apple has certainly given this much more thought and are likely many steps ahead of me. It would be nice if there were other companies that put so much thought into the future. At most, to spur competition and advancement… at the very least, to offer some kind of variety. If absolutely nothing else, companies will just follow Apple’s lead and the rest of the market will only be a few years behind what they’re doing.