You also have to think about the demographics of who all purchased and played 3x.
From all that I've read (and prior to now), D&D was at its most popular in the late 70s to mid 80s. That popularity was primarily focused among kids/teens/college aged people.
Fast forward to 2000. All those gamers from the 70s & 80s are now grown ups, never lost their enjoyment of playing D&D and many have disposable incomes. 3.0 is introduced. It fixes alot of the stuff this crowd didn't like (combat, multi-classing, magic item creation, etc.) with the previous editions and there is an OGL with the game. Wizards produces a bunch of products. Boatloads of independents pop up selling their D20 and OGL games. Just about every conceivable genre is covered from D&D to Star Wars to Conan and so on. D&D has its renaissance.
I suspect the above gamers, the ones with disposable incomes, are the ones who fueled the sales that kept the game popular this decade. I fall into the above group and so do my friends. When 3.0 was first released we picked up everything, WotC and 3rd party, that was published. Even if you find a current survey that has a largish percentage of younger players playing 3x, I think you'll find a parent, grandparent or uncle who played the game in the 70s/80s and introduced the kids to the current version of the game.
Now fast forward 7-8 years. Many of us have book shelves full of 3x stuff, boxes full of D&D minis (with 3x cards) and therefore have spent a sizeable chunk of cash. And we are faced with a choice of whether to keep playing the current version or move on to the new version. Alot will depend on how compatible the 4 is with 3x. Of everything I've read so far, it doesn't sound too compatible. So, invalidate the book shelves and the minis cards or keep playing 3x? Given my investment cash-wise, and the vast volume of 3x material I've yet to use in my campaigns...I'll probably stick with 3x. I suspect there are many others like me making this decision.
Through all of this, I'm still a huge WotC fan. I like what I've heard about the yearly additional PHBs, no dead levels class concept...both sound great! I understand (business-wise) why they have to do a new version. I really want them to succeed in order for the hobby to remain robust.
So will 3x survive? Much of it depends on how many independents keep publishing 3x stuff. New material keeps people interested (hence AD&D, 2e, 3e, 3.5e and now 4e.) The other factor is how many DMs out there want to keep DMing 3x. The hardest part of getting a gaming group together is finding a good DM. If the DM prefers 3.x and people keep publishing 3.x stuff, I suspect 3.x gaming groups will be easier to find than with previous editions. I don't know, maybe I need to PDF publish all the half-written stuff I have here.
I gave version 2 a miss, who knows, maybe I'll give version 4 a miss, but start playing version 5 when I'm in the old folks home?
Thanks,
Rich