Look again at this chart:
The projection of that graph is that tablets and netbooks outsell laptops and desktop PCs by roughly 2016.
You're the CEO of Microsoft in 2010. You currently have a great OS that sells really well on laptops and PCs and you have five years to get your company ready for the waterfall that your company is rapidly about to be swept off as the laptop and PC market vanishes from under you.
A) Introduce a new framework (like you did with WPF and Silverlight, which both failed) and hope it catches on so rapidly in the next two years that your platform is a solid tablet player by the time 2016 comes along and your laptop market dies?
B) Put it front and center (like you did with UAC) which will be really unpopular at first but will force developers to make the necessary changes to their applications so your platform can move off the sinking market of laptops and desktops and into the vast untapped wealth of tablets and phones?
If you answered (A), try again, but this time you're the CEO after the shareholders dismissed you for failing to have a 10-year strategy for your company.