Windows 10 is the last build of windows. there will be no more "new" operating sytems.
Windows 10 is a whole rebuild of the Windows operating system "again". A little history. Microsoft has really had 3 main operating systems. MS-DOS, Windows and Windows NT.
Microsoft Slapped a GUI on MS-DOS and called it 3.1. rebuilt it a few times and called it Win95, Win98, Win2000. Those copies are "home" user friendly, but not the most stable. Windows NT (networking) is not user friendly but rock solid stable. So they Merged it and called it windows VISTA.
The merge of home friendly and windows networking was a disaster in Vista. They plugged away at it and re-released Vista as Windows 7! A huge success. Now they gave Windows 7 a new look, called it windows 8! A disaster again Win8 is meh a marketing blunder copy? Who knows.
Now with everything techie going, streaming, high speed internet, 4k gaming coming next year, cloud services, tablets, and so on. Microsoft New Windows 10 will be a portable operating system.
They will continue to work on it, but the sections of the Operating system can be individually updated from Microsoft to support new technologies. Windows 10 is suppose to be truly a plug and play operating system. install it once and it's good for a 100 years. They just keep rolling out updates to modify the software to compensate for the changing market and technology.
This is why they skip windows 9 altogether. If it works as intended they really did make it to 10, the finish line.