As many has stated, M+ is what keeps them around. You might be able to argue the lvl or reward but its existence is important to a huge player base. WoW has always pretty much had some alternative ways of gearing besides raids. Its started with vanilla with pvp gear vendors which evolved to regular gear vendors and currency systems to obtain loot. M+ just removes the vendors and add rng to the equation. The ability to endlessly repeat them unlike the currency systems which had weekly caps might be some what of an issue but the content itself is not.