3.x isn't unstable, not even close. It also doesn't really have the problem that 2e did with too many suppliments. After all 2e books presumed you had every other D&D book ever written, and loved to reference them all, including ones that had been out of print for years or never had big print runs to begin with (which WotC has been reluctant to do). 2e was unstable at the core, and too limited to play a wide variety of fantasy settings without very heavy house-ruling, at least 3.x is flexible in this regard.
Also, the 3.x core is actually self-contained. I never, ever saw anyone be able to play a decent game of 2e from just the core books, they were so rigid and inflexible. You could play a game with just the core books, but it felt like it was stripped down and had most of its parts missing. Those optional books, like the Complete Reprints series, are just that, optional. Your D&D game will run just fine without them all, and the people here on ENWorld aren't fully represenative of all D&D players. People who come here are very likely to buy everything WotC makes, and probably a lot of 3rd party stuff too. However, the typical D&D player, in my experience is a player and not a DM, who buys just the PHB, and maybe a sourcebook or two related to his favorite type of character or setting. That and most DM's I've seen lately don't allow "Kitchen Sink" campaigns where anything goes, including anything WotC wrote. Just because it's "Official" doesn't mean it's going in my (or someone elses) game.
D&D isn't like a CCG, it isn't collectable, you don't need booster packs/new books to keep playing. Also, there are games of 1e out there still after all these years, I know a 2e devotee who runs it when he can, plenty of people still play 3.0, and you can bet hard cash that when 4e comes out there will still be lots of players playing 3.5 and not having a whole lot of interest in migrating.
I honestly don't care when 4e comes out, because I am perfectly content with my own personal home-brew of 3.5, with some holdovers from 3.0 and a healthy dose of Unearthed Arcana variants. I have the version of D&D that works for me, and I really couldn't ask for anything better.
If I had to say though, based on prior patterns I'd put the release of 4e as Gen Con of 2007, up to 2009 or later if D&D sales are high and they don't need to drum up business by re-releasing the core books.