What if Apple introduced workstation-class ARM another way; made a really powerful iPad/iPhone which could sit in a dock with keyboard/mouse/monitor, and run both iOS and ARM OSX?
x86 computers could continue to exist for high-end users, but typical-users might be content with a hybrid tablet/computer. As ARM increased in power, x86 might dissapear entirely.
Your comments on Windows compatibility/virtualization are spot on. Cloud streaming, Citrix/XenApp, can help in some situations here. In a few years virtually all major apps will probably be cloud/browser based (Microsoft Office, Adobe Suite, streaming games/apps, etc). Windows on Mac might not be as important then. It's possible Microsoft might release ARM Windows too (besides RT). If ARM gets popular in the datacenter you may see Windows Server 2015 ARM Edition. They've done this in the past with Itanium.
I think that it would be more likely that they'd just drop OS X in that scenario, and just have a beefed-up version of iOS with better keyboard and touchpad support.
The number of users who would actually benefit from OS X vs iOS, but would not be ticked off at an inability to dual boot Windows or run any legacy applications, is very small indeed.
iOS for ARM and OS X for x86 seems likely to remain in the future, but I do think that Apple could do a netbook or dockable tablet running ARM/iOS if they really thought there was demand. I'm not sure there is, but if the product was good enough they have shown in the past an ability to manufacture demand where it didn't exist previously...
What you're describing (a mobile device that docks into a workstation environment and powers the KVM) sounds like the dream mobile product designers have been having for well over a decade. I remember Canonical recently coming up with a concept of a phone that would do this (see: Ubuntu Edge[0]). That being said, the roadmap implied by iOS 8 and Yosemite strongly suggests that they're just going to bridge these gaps over the Internet. Google is also pretty clearly going in this direction, and given the ubiquity of Internet access (especially compared to available KVM terminals), I think this is the direction we're all going to go in.
It's been done several, dare I say many times. Badly, of course. I saw one at fry's where the base even had a more powerful CPU and when you docked it, the tablet was just the monitor... but you could access the tablet functionality while docked through something that looked like your typical telivisions "Picture in Picture" functionality. There was also a phone, a while back, that would dock into a video/keyboard in a laptop form factor. (Many phones these days, I am given to understand, have hdmi out, so this is largely a mechanical engineering problem. Well, that and making the software usable on both screens.)
So yeah, if it is, in fact, a useful form factor (and I have my doubts) it's in the perfect place in the technology curve for apple; the tech is all there, but nobody has implemented anything usable.
x86 computers could continue to exist for high-end users, but typical-users might be content with a hybrid tablet/computer. As ARM increased in power, x86 might dissapear entirely.
Your comments on Windows compatibility/virtualization are spot on. Cloud streaming, Citrix/XenApp, can help in some situations here. In a few years virtually all major apps will probably be cloud/browser based (Microsoft Office, Adobe Suite, streaming games/apps, etc). Windows on Mac might not be as important then. It's possible Microsoft might release ARM Windows too (besides RT). If ARM gets popular in the datacenter you may see Windows Server 2015 ARM Edition. They've done this in the past with Itanium.