And while I won't argue that Windows' "Modern UI" currently lacks apps, I'm going to also point out that every other UI started with a relatively low number of apps. I know some are claiming that Blackberry 10 will "launch with the highest number of apps than any other OS", but BB-10 is not an "all-new" OS and that even its core QNX started somewhere with zero apps.
I personally believe your definitions of the different systems--RT, iOS, Android--are overly simplistic; overlooking some of their primary purposes for existing. WinRT and iOS are very similar to each other as they are extensions of their parent OS--meant for easy use in the hands where most people really don't need the power of the desktop OS. Android, due to its independence from any desktop OS (including Linux) is more of a netbook OS with tablet capability--demonstrated by the most popular Android "tablet" is the ASUS Transformer; little more than a netbook with a removable keyboard. I'm not saying it's bad, but its poor showing in the actual tablet market *except* as a media-consumption-only device like the Kindle Fire shows that it lacks that critical infrastructure that made the iPad such a success and could lose big time to WinRT for exactly that reason--over time.
Windows 8 is essentially, as you say, Windows 7 plus RT--though I am happy to see that Microsoft did eliminate a lot of old, legacy code that brought its overhead down almost 50% as well as its footprint on the hard drive. By everything I've seen in testing it myself and read online, Win8 is fully functional and notably faster on hardware even dating back to '07's "Vista Ready" machines. Now, without actually trying it, I wonder how well Win8 would run on those Netbooks that came out between '07 and '09--when the iPad shut them down?
Keep Up with TechRepublic