Greg Street, one of wow's ex developers who left WoW after the MoP expansion (i think) has said the sub based model is outdated for mmo's. It's a barrier for entry for new players because there are some days when you only want to play WoW for few hours but in order to do that, you are forced to pay 15 bucks for a sub. There are times where you are not playing wow for weeks so that's money ends up being wasted.
He's working on the new mmo game for Riot so we have to see how successful that game is but do you agree with his statement?. He plans to mainly make money from cosmetic items from the ingame shop e.g. weapon skins, hero skins, mounts etc while the game stays Free to Play.