Microsoft sells a boatload of copies of Windows.
They don't sell a lot of MP3 devices. Or tablets. Or phones. But they sell a lot of copies of Windows. And a very strong OEM partnership market.
With Windows 8 expect on the low-end they ship 200M copies its first year. On the high-end, think 450M. (they did 350M for Win7). This will be the default UI for all tablets and probably laptops (maybe desktops, but desktops are increasingly niche devices).
You're going to have a huge market of people now getting touch devices because the default touch experience is actually really good. Sure there's the other experience, but people will be able to largely stay in the default touch experience while doing consumption. They only pop out when doing creation. And those are the times on the iPad that you would typically go get your laptop anyways.
And in terms of the appstore... when there are 50M machines using this OS the first month -- there will be apps. Non-Apple devs will love to have an app store ecosystem of this size.
To put it another way, Gruber would be absolutely right if they had tried this with Windows Mobile. WP7 would be held back due to it, and WinMo had no marketshare to speak of. But for Windows proper this is actually the right move. It actually harkens back to them shipping IE with Windows and catching and passing Netscape. This is old fashioned MS leveraging their huge market position. It's something they frankly can rarely do anymore, but I think it will actually work for them this time -- maybe the last time.
And lastly, note that the legacy experience is really only there for Intel based processors. For ARM there will probably be very few legacy experience apps. I'm thinking Office and maybe one or two others. In a typical world, think Honeycomb, those tablets never build an app ecosystem. But in this world they get the huge installed user base of the Intel platform for app devs to target. So in one generation you may well have a solid app eco system completely sitting outside the legacy experience. They solved the chicken vs egg problem.