I'm currently unsubbed and would definitely stay unsubbed if the next expansion is Burning Legion. They've already done it, multiple times and dressed it up as many different ways as they can to try to make it more interesting and it just isn't. It was cool in Warcraft 3, it was cool in the Hyjal raid, the legion-related quests in their respective zones (namely Ashenvale and Desolace) are pretty cool, it was alright in BC...but no more. It's played out. Badly.
Besides, all that's left for us to do Burning Legion-wise is kill Sargeras which would probably mean the end of the game anyway, so screw it.