It's very difficult to tell the long term cause and effect of such things...all we can do is pick a specific point in time and look at what's happening then...
WoW did vastly increase the market...and it also increased the number of companies making MMOs wanting to jump on the bandwagon...
Although the majority were and are clones today...so it seems as if WoW has been detrimental...who's to say that the millions of players WoW brought to the genre won't start demanding something new...pushing companies to raise their game and ultimately driving on innovation...an innovation only possible because of the 8 million bored WoW players.
A wise game company would see the potential millions of players, pay very close attention to their requests and do something about it.
Without WoW, MMOs would still be seen as a niche and high risk type of game...the WoW-clone phenomenon seems like a necessary part of its evolution.