Re: The question of why
I agree with several things you said, but I disagree with others.
You certainly are correct that iOS is Unix inside. Android isn't exactly a Linux distro though, because Google replaced the libraries inside Linux with their own proprietary ones. What that makes it, I'm not entirely sure.
Does Microsoft need a new OS? Well, perhaps not. But if it runs different software, then it is a new OS. It doesn't matter that iOS is basically OS X inside. It's a different OS because it runs different software (and runs on a different CPU). It's not just the kernel, Mach, but an entire OS, named Darwin, which is OS X without the GUI desktop. Apple offers Darwin as open software.
But in Microsoft's case, Win 8 is two OSs bolted together. They run entirely different software. Their programming models are different. Their operating models are different. That makes them different.
The Pocket PC wasn't even Windows, just a facsimile. The OS was based on CE, which bears no real relation to Windows. Microsoft has been using that "Windows Everywhere" marketing ploy for years, event thought the different "Windows" OSs were actually different OSs. Microsoft felt that if they were called Windows, and looked like Windows, even thought they ran different software, people would be fooled. And in most cases they were, at least until they used them for a while.
Of course, all of those small Microsoft devices used a stylus with a resistive screen, no touch available, unless you regarded a hard tap with a fingernail as touch. Even the horrible convertibles were resistive, though they had 13-15" screens. They were still impossible to use.
Trying to use the Desktop on a Surface Pro is a frustrating experience. And now we hear that they may be coming out with a cheaper 8" version. They must be trying to torture people.
Oh, and by the way, how is paragraph support managed here? I've never been able to figure that our.