Floydy... I can see what you're saying, but the Windows development history can't be really summed up in that way, the NT-based OS series was pretty much a separate product that was developed more in parallel with the home versions of Windows (3.1, 95, 98 etc).
Windows 2000 (released Christmas '99), was MEANT to be Microsoft's grand attempt at converging the NT and consumer versions of Windows together into one operating system that could be used in the home and the corporate environment.
Unfortunately this didn't quite work out as planned, as too many applications were incompatible with the NT kernel and relied on quirks in Windows 98 to allow them to run, so Windows ME was quickly released as a 'stop-gap'. It incorporated some features from Windows 2000 (notably the networking stack, and the new shell/colour scheme), but was still based very much on Windows 98.
Purely speculation here, but I wouldn't be surprised if Windows ME was never really on Microsoft's development roadmap until they realised that Windows 2000 wasn't going to be this "one size fits all" system that it was meant to be. XP finally managed to do this, probably because application vendors had been working hard to get their apps compatible with Windows 2000, so by the time XP came out, the world was a bit more ready for it.
Windows 7 will likely be to Vista what XP was to Windows 2000... another few years and we will all have better PCs that will be capable of running Vista, and application vendors will have sorted out the majority of Vista issues, so Windows 7 won't seem like such a monster and the world will probably embrace it a lot better than they did with Vista!
1990-93: Windows 3 (3.0, 3.1, 3.11)
1995: Windows 95
1998: Windows 98
2000: Windows ME
The NT product line was something like this (ignoring NT 3.1 and 3.5 which were released sometime before Windows 95)
1996: Windows NT 4
1999: Windows 2000 (NT5)
2001: Windows XP (5.1)
2007: Windows Vista (6)
So who knows when Windows 7 will be released. They are currently saying it'll be ready for Christmas 2009... other reports I've heard state mid-2009, and mid-2010. I think they're keeping their options open by confusing everyone on the internet with different release dates!!
I would certainly be expecting a release long before 2012-13 though.