Wow has been out since 2004, and has been the de-facto MMO for atleast 5 years and made 100's of millions of dollars. I would hardly call that fail.
Just like long running TV shows, even MMO's have to reach an end-of-life. While players associate the MMO with an alternate reality for themselves, companies with investors are looking at products with 'growth'. So Bliz is known to be working on a new MMO, and being the leader in the market, anything they put out is going to cannablize thier own customer base.
WoW's huge player base has somewhat streached the typical MMO/Game cycle of Release-expansion-maintenance-closure but it's not immune. And of course Blizzard isn't going to shut-down the cash cow while it's still pouring out milk, it's looking 5-10 years down the road at the next game that isn't based on subscriptions, since the MT model has proven to be much more lucrative.
That is, Blizzard isn't looking to create antother Iconic MMO, realistically they'll put their development efforts into high-profit games like Synga and let WOW just slowly fade away.