There is currently no reason why they can't have apps written for WP7 running on Windows 8 tablets, or at least make it easy to port existing XNA and Silverlight apps over. WP7 might have a nicer UI, but it seems like MS is giving Windows 8 full touchscreen support.
I get what you're saying. I really do. But we should talk. In order to understand the broader market (outside of geeks buying gadgets for themselves), you have to see past your own interests. There are plenty of people for whom hacking on their computer is a pastime. This can mean any number of things like building your own computer or building your own software. The key is, hacking on the computer is an end, not a means. In the broad market, the computer is a means, not an end. Full stop.
Most business users want to accomplish a goal. In the business space, it's about accessing, sharing, and manipulating business data. The reason everyone (outside of geeks) says they want a full blown OS on a computer is because they want access to the tools they're used to using for accomplishing these tasks (hello, Excel). As Henry Ford said, "If I had asked people what they wanted, they would have said faster horses." In essence, Microsoft is breeding faster horses while everyone else is designing cars.
I'm not sure what it means to be an "open platform advocate", but when I hear that term, I think open source software, and to use Windows in that same sentence really blows my mind. That's why I'm left to assume that you're talking about open in the other sense (open access to the system). For my part, I'm an advocate of open platforms in this sense, but I have a sense of scope. I realize that the tablet market for business managers doesn't overlap entirely with the tablet market for hackers.
I think that as the computing market continues to mature, devices for hackers will become a niche. This is a natural consequence of the fact that hackers are a small group. We are not in the majority. Be thankful for that, because the demand for hackers is high and the supply is limited.
The days of the enterprise market leading the consumer market appear to be over.
That is, unless Microsoft comes out with a radically new version of Excel which every business just has to have. ;)