They tried that once already
Remember Longhorn? The project that got restarted after 4 years of development and eventually became Vista? Its first iteration used the CLR extensively for OS features, but it just didn't work well enough. Some pieces (WPF) got released separately, but MS won't make the same mistake again.
It's hard to say whether the new cross-platform (hardware platforms) strategy will work. I don't see the demand for Windows on phones and tablets, at all.