Windows Blue is coming, sooner rather than later, and it’s internally numbered as 6.3.
Apparently no one can actually update the major version of an operating system anymore. This is what happens when the low hanging fruit is eaten and no new trees with new fruit even grow up.
But the worst of all this (an artificial reason for keep the version numbers from progressing even if there were merit):
But the official reason is much more plausible and depressing. Broken apps include version checks of major and minor version and barf if major version is >6. Some barf if major.min is >6.0 or 6.1, of course, but this is apparently less common.