What does whatever kernel underlies the phone have to do with anything? Current gen WP7 devices would be perfectly capable of running an NT based phone OS, the hardware is easily powerful enough. And WP8 will run all WP7 apps, so again that's a non-issue too. The software side of things is clearly not the issue here.
I suspect the reasons behind Microsoft staying quiet on this one ultimately comes down to the carriers. They don't want to update people's existing devices because it makes it much harder to sell them new shiny ones. As has been evidenced by the WP7 update story, Microsoft wanting a clear consistent update pattern has been well and truly slammed by the carriers decisions.