I'm guessing it was a decision made at an executive management retreat. And alcohol was involved.
If you recall, they wanted to get on the "rapid release" train, and Windows "8.1" was probably supposed to be Windows 9. But, you know, that pesky reality and all. So now they think they can catch up just by bumping the number.
The real problem here is that Microsoft has run out of anything useful to add to their OS. So now they are running around in circles trying to find ways to take things away, or fuck things up so people think they need yet another version.
Honestly, bumping it to "10" makes no sense whatsoever. If they wanted to give it some weird alternative fancy name like "Selenium Edition", ":P", "Xista", or whatever, that would at least make some sense. But this is just... stupid.
All "Windows 10" tells me is that they can't count, and we shouldn't count on Microsoft.
For now I am going to call it "Windows Can't Count Edition".