Videogamer.com has an interview up with Ryan Barker, the lead designer for seminal MMORPG EverQuest. When asked if he thought WoW would exist without EQ having been around, he replied that he didn't think so, and that Blizzard designers would likely agree with that statement.
I think he's right, too. The success of EQ allowed for countless imitators and innovators to follow in its footsteps, and WoW is certainly both of those things. What made WoW successful in the first place, beyond brand recognition, was the fact that the developers -- whose team consisted of a number of former EQ devs and prominent community members! -- refined and added on so many features cribbed from EQ. They made the formerly hardcore-only genre accessible to a wide variety of players and age groups, and in doing so broke subscriber and sales records -- thus continuing to make new MMOs financially plausible. And with WoW's improvements to the diku formula, the genre is now filled with WoW imitators as well. History repeats itself.
Sure, it's entirely possible that WoW could have existed without the advent of EQ, but it would have been a very different game if it existed at all. And I doubt it would have been anywhere near as good without having been able to learn from EQ's myriad mistakes or study its successes. We owe a lot to Old Man EQ. Now get off his lawn.