The actual version number of Windows 8.1 is 6.3.
Looking back, it was like this:
5.0 Windows 2000
5.1 Windows XP
6.0 Windows Vista
6.1 Windows 7
6.2 Windows 8
6.3 Windows 8.1
8.1 really is still on the same overall core as Vista, and the actual internal version number (you can see it by running the command "ver" on a command prompt) reflects this. It's not clear yet whether 10 will actually be 6.4 or 7.0; my money's on 6.4.
It's also worth noting that the system requirements for Windows havn't gone up since version 6.0 (Vista), another reason to believe that "Windows 10" will actually be version 6.4.