Yeah the only thing is they're now vested in hardware. Their goal moving forward is to have first party hardware like the Surface occupy 50% of all Windows device sales, and OEM hardware the other 50%. Office is huge for them - it has the highest profit margins and they can leverage it as a product differentiator to generate sales of their own platform/hardware. Meanwhile if they stick it on iOS or Android, they automatically lose 30% of their profit margin along with product differentiation. I expect the iOS/Android versions of Office to always be crippled compared to the Windows version.
You're probably right but I don't really see any evidence that iPad and Android tablet sales are declining in favor of Windows RT/8 devices. I don't think people care that much about Office. There are other options that meet people's needs on a tablet. And if Apple goes the free route with their iPad apps even less reason for an iPad user to go Windows. Office will always rule the roost on desktops but tablets I'm not so sure. Still I think MS would get enough subscriptions to make it worth their while.