My thought for why people have quit? Lack of engaging content this expansion. Sure, questing really is better than ever. They've come a long way since vanilla. But as far as end-game, they've fallen off this expansion. I think they've streamlined a lot of things very well. For example, making 10 and 25 mans both rewarding and viable, etc. But the lack of new, and well done end-game content whether it be 5-mans, or raids this expansion has been pathetic.
Also, as far as the accessibility part, I kind of get what they're saying. The gaming world is changing. A lot games are taking the free to play route and with that, they try to make their games easy to pick up and play and enjoy. WoW is not easy to pick up and play for a first time player. And by the time you're 85, you have so many abilities it's almost overwhelming. For someone who's never played an MMO before, playing WoW and getting good at doing it isn't something that can do quickly, and without a lot of time researching and learning. It's not easy to just pick up, play and enjoy.