4 Tiers of raiding prooved that exclusivity is "terrible" for WoW.
Id rather have 4 raids, than 4 tiers of raid any day, the issue lies in the fact that the more 1% everything needs to be the more blizzard needs to design everything around exactly that, 1% of the playerbase.
Leaving the remaining 99% unattended and uncared for, it was a massive mistake to even -think- that an entire expansion that lived on raiding alone would be a success.
So if WoD prooved anything, no, exclusivity is a TERRIBLE idea.
Id say however playing devils advocate that the problem lies in the fact that there are too many options on either side, blizzard themselves said this:
LFR is for people that just want to experience the raid at a lazymans pace and have IRL commitments.
Normal is for friends, family, casual raiders that want to play the game for fun.
Heroic is for gearing up elite raiders for the end game content, and is there for training raids to progress properly with hardcore effort.
Mythic is for the elite.
Thats, the problem.
You have 4 tiers of difficulty, catered to too many variables of people. It was -simpler- and -better- when there were only 2, tiers. One suited to people that play at a casual pace, and one suited to people that -want- to do the hardest content.
Because of that, the main content, world content, pvp content suffers as nearly -all- the content is being developed around 4 tiers of difficulty for both raiding AND Dungeon content.
Now to be fair blizzard has SORT of got this right in Legion in which certain dungeons can ONLY be done on Mythic difficulty, but its not quite the fix we need to make that content efficent again.
The reality is much simpler, we -need- progressive content in quantity, not quality. Having to do the SAME thing repeatedly on a higher difficulty is fucking, boring and most of us are DONE with that format.
Theres a reason most stick to Raidfinder or Normal, because most simply get burned out once they get past that, because even if you defeat the best bosses, your STILL doing the same fucking shit.
This is the issue, right, there.
And as a result, it needs to go, remove 4 tiers, go back to 2, stick, with 2.
And make them effectivly balanced for both parties, Casual for those that want to play at a relaxed rate with less focus on challenge, and Hardcore for those that actually -want- to get the best loot, gear and general bragging rights.