I believe that WoW's impact on the growth of other MMOG userbases is so miniscule that it may have taken more experienced players away than its contribution to those bases with new MMOG players that started with WoW. I can't back this up with empirical data, but I can state the obvious that WoW has had a crazy averse effect penetrating a mainstream userbase and that most other MMOGs haven't really seen a huge numbered growth since WoW launched.

A few MMOGs shut down this year, you would think a game like Auto Assault, which was definitely niche, would at least sustain enough subscribers to thrive and yet it flopped. Other MMOGs like CoX are more or less stagnating. I think the only real break-out hit so far this year is LotRO, and if they were expanding like crazy they would be adding servers, but they may have peaked already. I would say CCP’s userbase success in subscriptions comes from the fact that yes, it dominates the MMOG space market, and also that they expanded EVE into (million+ player potential, PvP freaks) and the recent client additions.

I argue that the reason other MMO's aren't seeing an increase in numbers from WoW is that they are so poor quality in terms of enjoyment that they compare extremely unfavorably with WoW.

Only Blizzard seems to have the magic formula of fun/grind that makes players continually interested in their world.