No matter how people try to twist it and turn it, purely speculating or giving biased opinions on why it's a success and not a failure, the numbers are there and they paint a perfectly clear picture.
MMO-C posted data on raid participation four months into Siege of Orgrimmar and Blackrock Foundry. I would have preferred comparing last tiers, but for some reason the respective data doesn't seem to be available for four months into Hellfire Citadel:
This one was posted 4 months after the release of SOO:
http://www.mmo-champion.com/content/...Tweets-Fan-Art
On average, 3.277% of all active players killed the respective bosses on the hardest difficulty.
The three access bosses were killed on average by 6.29% of all active players (Immerseus, Norushen, Protectors).
This one was posted 4 months after the release of BRF:
http://www.mmo-champion.com/content/...metra-Gameplay
On average, 2.834% of all active players killed the respective bosses on the hardest difficulty.
The three access bosses were killed on average by 4.58% of all active players (Beastlord, Oregorger, Gruul).
That's 28% less players that were active killing the first three bosses and 15% less players that were active clearing the respective raid after four months.
I'm not at all convinced that this is the result of the intended "tighter tuning" but rather the result of artificially gating content accessibility behind the improved attendance boss.
And let's be clear: If we had the same data for Hellfire Citadel, things would look even worse. Naturally, participation rates decline the longer an expansion is out (see here:
http://www.mmo-champion.com/threads/...ghout-Warlords). I'd assume we're talking 25% less active players clearing the hardest difficulty in the same timeframe compared to previous expansions (general raid participation drop BRF to HFC was roughly 40%, see thread above).
Add on top that raiding has been the only real endgame for WOD and you just can't deny the fact that 20m Mythic has been a failure—at least if the goal wasn't just to make it more exclusive with the help of a lazy logistics barrier.