Nonetheless, UTF-16 will most likely stay. They might add wrappers around all API functions that convert arguments first, so we'd get CreateWindow8 alongside CreateWindowW and CreateWindowA, but I wouldn't hold my breath. The duality of API functions was to enable programs to be built for NT and 9x with the same codebase. It was never really intended for people to call them directly to enable non-Unicode-aware applications on NT.
The file system cannot migrate from UTF-16 anyway without breaking things, so that will stay as is.
IMHO there's pretty much nothing to gain, except a lot of risk in trying to change Windows to UTF-8 internally and the current interface might be an inconvenience to some developers, but not as much as to justify changing the whole OS.