Sorry but if the richness of the ecosystem is the main reason for change, then the obvious answer is Android.
XNA is actually a decent toolkit, and the Visual Studio environment is way better than the Android Eclipse mish-mash.
What's preventing uptake is a complete lack of understanding in their market, an inability to take down even weak competition like RIM, and no clue how to leverage their success from the 360 on to a phone.
If they'd just called it the Xbox Phone it would've sold ten times better.
It's quite obvious that the Windows brand is burned; people hate Windows -- and Microsoft know it better than anyone else!
They had a campaign some time ago where they tried to demonstrate that people actually liked Vista, if only they didn't know it was Vista.
Of course, what it really showed was that people are incredibly prejudiced against the Windows brand.
Even as an Apple fan, I always thought the 8-core Mac Pro was expensive compared to the alternatives, yet the Dell equivalent is about $800 more.