A great MMO cannot happen without a subscription and is a major reason WoW's on top of the MMO world. It is a major reason but not the only main reason, timing and product is of course another consideration.
To keep a MMO with good fresh content takes money, to do it very well takes even more. The sort of money that only a subscriber base can fund.
Past experience has shown me, that when a game comes out with no subscriber base funds, using only in-game shops to fund the process...they are not that great. WoW has had the money to keep up on the content and quality.
When a MMO has to move away from their original subscription fee to free to play, that is when it is failing. I have been a critic of WoW at times with some of their choices but I like that they intend to keep that subscription fee going to maintain their game and that they are in a position to do so.
Once you move away from that, the game has failed.