Logically the older versions will remain to be maintained until MS is releasing a Windows solution that is perfectly backward compatible down to XP - the original idea to just "update software as required" was a bit of a naive dream from Microsoft since a lot of software developer have deceased and businesses are stuck in place especially those with highly specialised software - Microsoft has to care about that.
Quite honestly, when they relaized they'd "hit it out of the ballpark" with win2000 and then XP their best bet would have been to continue improving that codebase instead of launching off on an altogether new setup.
I would really have loved to see specialized versions of XP for multimedia, one for small office, one for netbook etc. They would also have been a lot smarter to introduce Aero as a replacement shell option for XP instead of trying to reinvent a perfectly good wheel...
The reaction from XP to Vista wasn't that Vista was crap (it was but that's not the reaction) it was that it was in your face, continuing Microsoft's trend of increasingly treating their users as idiots. I could have answered yes to all those "Do you want to..." prompts if they weren't so outright insulting. Similarly the reaction from Vista to Win 7 wasn't that Win7 is better (it isn't) but rather that people could once again use their computers without constantly begging permission.
A couple of years back I got busy with NLite and gutted a copy of XP, right down to bare minimums. I gotta tell you it ran slicker than loose through a goose... I use it today on my HTPC and Netbook... Atom processors that keep up with my Win7 AMD-X2 systems... Personally I think M$ would have scored a much bigger hit if they'd come out with a stripped down version of XP, with only the bare essentials and then made the rest into paid add-ons... Don't need CD burning? Don't install it? Don't need zip folders? Don't need 370 languages? etc.
Frankly I'd have to say that for performance and ease of use, Microsoft is definately going in the wrong direction.