Yes, it's probably in a decline and is nowhere near 12 million subscribers like Wrath had, that was quite literally an almost impossible feat and achievement for WoW and should be commended.
But if you continuously tell Blizzard that the MMO market is dying and that's your reasoning for why WoW is in decline, you might as well be telling them to stop investing money in WoW. They are a business after all, and if they think a market is dying off, why would they continue spending money on development of a game for a dying market? If you care about the game at all, stop saying this.
If you think that WoW is in a decline because of some bad game design decisions, then you still believe the game can be turned around with some better game design decisions and not be in a decline anymore.
Which of these opinions do you think is more helpful for the game? Which person do you think cares more about the game, the one who thinks the MMO market is dying and WoW is in decline because of it and there's nothing that can be done to save it, or the person that thinks some better game design decisions can still turn the game around?
Pick your battles wisely.