according one microsoftie on Reddit it's because many apps perform a look up for 95/98 versioning with 'Windows9*', that would trigger compatibility issues. It's the same reason why NT kernell has stayed on version 6 since Vista, otherwise if it were changed to 7 it would break most software which looks for v 6.x
Then according to Paul Thurott Microsoft named it Windows 10, because they wanted more distance from Windows 8, a symbol of the next version being a big upgrade or different from Windows 8.x
New rumor has it that MS may push the official release to May or June 2015, because the actual work is bigger than what most people think.
I'm on Windows 10 as my main OS and so far it's stable with no issues.
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