A lot of industry insiders have argued that World of Warcraft has been good for the MMO industry because it expanded the overall market. The idea is that players start with WoW, and eventually get bored with it and start looking for a new MMO fix. In other words, WoW is a"gateway" MMO. On the surface this seems pretty reasonable. Certainly WoW has a much higher profile than any previous MMO. And the market has clearly grown in the past four years.
At the height of the "golden age" of MMOs, EQ had roughly 500K subs and DAoC (the most successful competitor of the day) had roughly 250K subs. I'll be generous and pretend that UO had 200K subs at that time. So that gives us a total market of around a million subscribers in mainstream MMOs when EQ was king. In the post WoW era, there are very clearly more MMO gamers. EVE and WAR have roughly 300K subs these days, and FFXI is still hanging tough at 500K subs (amazing). Those three alone have more than a million players. Even without doing any ass math (i.e., math using numbers pulled out your butt), I think any reasonably informed observer would agree that the total non-WoW MMO market is at least several times bigger than it was before WoW launched.
However, I think we would have gotten here with or without WoW. The Korean market and the explosion of micro transaction MMOs have obviously been huge boosts to the industry. Even ignoring those factors, I think there have been several other "gateway" MMOs over the years that haven't gotten much credit. MMOs that reached out to new markets and have a gentle initial learning curve.
Everquest Online Adventures never had many subs, but it was a first MMO for a very high proportion of users. When Sony shipped the modem for the original PS2 it came with a demo of EQOA. That got a lot of console gamers to try an MMO for the first time. City of Heroes likely expanded the market by moving out of the fantasy genre. It also remains much more rewarding at low levels than is typical for an MMO, you will feel like a badass within the first hour of play. This has to be better for new users than the tepid starting pace of most MMOs. More recently, LoTRO seems to have attracted a lot of new MMO fans due largely to the IP. Almost everyone I meet in game is either a EQ era MMO vet or a new MMO user. Those who cut their teeth on WoW almost always seem to revile it.
Which brings me to my last point in this rambling post. I find Syncaine's ongoing tirade against "WoW tourists" more than a bit pretentious. However, I think he is right about one main point. The majority of WoW players that try other MMOs don't like them as much as WoW. The vast majority of MMOs aren't as fast paced as WoW, have much steeper learning curves than WoW, and don't run as smoothly on a low end PC. While I have zero hard data to back this up, I strongly suspect that the bulk of players that get bored with WoW leave the MMO market altogether. Assuming that's true, WoW isn't really a gateway to other MMOs. It's more of a separate market from other MMOs operating in a vacuum. WoW is neither helping nor hurting the industy at large. I suspect that the real "gateway" MMOs are modestly successful MMOs that appeal to previously untapped markets (e.g., LoTRO and CoH) and ftp MMOs with a lot of younger users (e.g., Runescape and Club Penguin).