So the vicious cycle continues
It appears that Windows 9 will be "Windows 8 done right", just as Windows 7 has been "Vista done right". Hopefully, Windows 10 will break that cycle, but time will tell. Just as it did with DOS, MS has fallen into the habit of releasing stable, well-received odd-numbered versions of Windows, while even numbered ones end up with major problems; Windows 95 (Windows 4.0) and Windows NT 4.0 appear to be the only significant exceptions to this rule.
It is helpful that Satya Nadella seems to understand the difference between an OS and a UI, especially that the former can have more than one of the latter; the tendency at MS throughout the history of Windows has been to conflate the two.