"Merging" the desktop and mobile-touch universes is a fool's errand. They shouldn't be unified for the very same, very good reasons that the iPhone was served by a fork in the first place.
Frankly: you're looking for a fundamentally different experience at the desk top than you are on a mobile. And ARM is a bad choice for the types of tasks that people are still buying traditional PCs and Macs to do.
I don't think it's really a matter of desktops becoming like mobile devices. I think it's "desktops as we know them will be going away (except in specific use cases)".
My wife's Macbook has seen better days. When it finally dies, we're going to replace it with an iPad, a bluetooth or docked keyboard and a camera connection kit. It does everything she needs. I imagine this will be the same when my kids get old enough.
And why is ARM a bad choice? Have you heard about performance complaints from iPad users? Have you seen Rage HD on iOS yet?
I tend to agree with that. But that's a wholly-different argument than suggesting Apple would make their desktop OS ARM-only.
OS X is Apple's solution for those use cases where desktops do and will persist for the next decade, at least. As such, making it ARM-only would require the use of currently-inferior chips, chipsets and tools. It would murder Apple's steadily-growing share of the very market OS X exists to serve.
iOS is already serving their mobile devices, and doing it very well. It stands to gain very little from a hypothetical OS-merge and stands to lose quite a lot by becoming more PC-like. (simplicity, stability, performance, etc)
It may be true that the current ARM offerings are inferior in terms of benchmarks, but as far as general computing use they're probably overpowered. I don't really need an i7, a Core 2 Duo is fine. Nearly everything seems generally overpowered these days.
To you maybe. Some of us also do stuff like Video and Audio editing, processing and mixing on our machines, not to mention photoshop and stuff. Remember, the industries that made Mac cool in the first place? yeah, we're still here, just getting addressed less and less. i everything is nice, but there's still millions of people who use this shit professionally.
Apples attitude - i.e. killing the xserve when there's hundreds of the things in usage in render farms and video asset serving etc. - is, while typical, somewhat frustrating.
Apple's "attitude" is called meeting market demands and eliminating the cruft from their product lines. They're here to make money not do you (or me) favors.
I'm not sure how far towards the high end ARM will go, but it sure makes sense to have things like Apple TV on ARM. It uses less than 6 watts.