There's more to an operating system than the kernel and drivers. The API for instance. And security - which in the case of phone-style devices is a hugely significant issue. Sure, it's possible to create a new shell, but that too is only a little part of the overall challenge. Then there's all the tools, applications, software distribution, DRM... If it were that easy MS would have done it.
One of the reasons (as far as I'm concerned) that Apple don't do 'full' multitasking is that the more work that's done, the more power that's drawn. Allowing unfettered access to the full multitasking functionality of a full-size OS such as Windows (or MacOS for that matter) is asking for trouble.
Most developers are rather average and very selfish. Whilst you may be capable, most aren't. Even if most developers are capable, the fact that *some* can screw it up for the others is a good enough reason to question the OS architecture that allows them to create poor code in the first place.