I have to disagree. Blizzard has never faced competitors as strong as it will over the next two years. Previous competition has consisted of things like Warhammer Online and Aion, which simply weren't in the same class as WoW. If you compared the fundamentals of Warhammer to the fundamentals of WoW, WAR simply came up short, and as a WoW player you'd be hard pressed to say "well, this game is suckier than WoW but has nice PvP, so I think I'll ditch WoW forever and stay with this." WAR did PvP well, but that was it. Aion was pretty, but grindy and not well rounded. Lord of the Rings Online was beautiful, but slow and low magic. All these games offered something different from WoW, but they couldn't really compete with WoW on it's own merits.
That's changing. I think Rift has demonstrated that it can stand toe-to-toe with WoW on features, gameplay, and quality, and surpasses it on graphics (subjective of course) and developer interaction/content additions. The things that WoW has are an installed playerbase with years of investment in their characters. That's a very strong tool of retention, but WoW players are running out of things to do with those characters much faster than Blizzard is creating content for them. As time goes on more and more WoW players are going to realize that rather than level yet another alt, they could level a new character in an entirely new game and get a new experience. Many of those players will return to WoW for the next expansion, but some of them won't. Over time, WoW will continue to lose players this way and gradually fade into obsolescence.
Rift alone won't have that much of an impact, but WoW isn't facing just Rift. Both Star Wars: The Old Republic and Guild Wars 2 are coming. Both are high profile, big budget titles by proven developers. Both are generating a lot of hype and interest from MMO players, including bored WoW players. Of course, people said the same things about WAR and we know how that turned out, but I don't think we're going to see WAR-style over promising and under delivering in this generation of AAA titles.
Is WoW doomed, is it going to be "killed" by these games? No, of course not. Even if the slip in subscriber numbers right after the release of a major expansion is a sign of things to come WoW is going to be around for years. Due to the slow churn of players I'd be surprised to see WoW drop below 10 million subs for a few years yet, and it's decline might take a decade. However, I think we've seen the beginning of the end of WoW's total market dominance. Unless they pull off something utterly fantastic with their next expansion and get it out sooner than 19 months from now I think many WoW players have passed the breaking point and are going to move on to newer (and greener) pastures. Just how much boredom does Blizzard think people will pay for?