Look at the biggest Android devices: Moto Droid and HTC Eris, etc. (Nexus One is not that huge...yet)
The most popular ones have some sort of customization. Moto's BLUR is going to be huge. HTC Sense is very popular.
Point is, OEMs have a choice: do a "Google" phone (basic OS) or a custom version (Sense, BLUR, etc.)
It's working
very well for platform, no? Why would MS take
years of OEM customization away from them? The ability to customize the UX/UI is a way for an OEM to fly their colors, make themselves standout, define their brand. You take that way, they are all stuck riffing on the same tune. It's exactly why WinMo was so boring when Wm5 came out; only Palm was seriously hacking it.
I've personally spoken with Microsoft before on this topic: they love HTC's Sense. They think it's a selling point for their OS. This was in October and I find it hard to believe they would do a 180 on the topic.
All evidence I have suggest MS is doing the Android route: have
both a Zune UX version and one that can be customized. Heck, they even mention they will be making
custom UIs for carriers. I think OEMs would be ticked "Oh
you can do custom UI's, but
we can't?"
There may also be a Zune-type feature phone (very locked down, guaranteed performance/experience).
Going further, if the HD2 is getting the update (and I'm confident it will), do you think HTC is going to let users go from their Sense 2.5 to "X"? You don't think that would, you know, disrupt the customer experience?
That's my bet.