Not likely
Every game that's come along to challenge WoW has failed spectacularly because they target that niche you were talking about, not just in terms of hardware, but also the type of player.
They've tried to be the hardcore players' best friend and ended up alienating the masses.
You correctly pointed out that Blizzard's success is that they managed to produce a game that be as complicated or as simple as you, the player, want it to be and over the years have refined it to a very polished and very stable game.
It's hardly perfect, no software can be perfect, but down time is limited to 1 weekday a week, usually at times when the majority of us will be working or studying. The rare occasion where that downtime encroaches onto the average person's game time usually results in additional days being added to your subscription.
But the biggest part of why WoW is so popular is for once we actually have an online game that appeals to women almost as much as men. Don't know about the rest of you, but simply playing and interacting with both men and women is a far more enjoyable experience than playing with a bunch of anal schoolboys with no life.
Doesn't mean that sort of person can't be happy in WoW too, there are many of them here, but regular people are catered for just as well, and best of all, most of the advanced and best content is accessible to the rest of us that aren't hardcore players.
This has taken time, and any new game would need to be given a chance to get that polish and stability you talk about if you want to be fair. But when they bring their systems down during the most popular times of day and days of the week to play an online game, when their patches and fixes make the game worse (or in the case of Conan, literally unplayable if you choose the 'wrong' type of character) they're hardly endearing themselves to the type of casual player they'll need if they want millions rather than hundreds of subscribers.
Also no other game has incorporated the social aspect of WoW so flawlessly. I talk about playing with men and women, and how that's a far more enjoyable experience, however if the social tools weren't there to support this, it just wouldn't matter who was playing.
It's not that other games don't have the same features, but rather that they're so intuitive within WoW, to the point where you don't have to think about how to find a friend or communicate using voice over ip software, it just works.
During the 2 years I've played WoW I've given 3 other games a chance to win my approval, mostly because I enjoyed the genres they belonged to. But the simple fact they're exclusively populated by the very people I don't particularly enjoy playing with, the kind of hardcore player that talks in some weird, alternative internet language of their pwn, has put me off all of them pretty much immediately.
The people I play with come from all walks of life, from firefighters to construction workers, office assistants to network administrators, men and women with real lives and families and who understand that I can't spend 8 hours at a time inside a game world if I want to remain married. These people simply wouldn't set foot in any of the games that called themselves "WoW killers" and that will probably hold true of the next year's offerings too.