At this point, the only expansion that can directly be connected to the sub drops of its period is Cataclysm - it was such a massive difference from Wrath and Blizzard recorded and acknowledged it as responsible for sub drops. And Wrath was the high point in subs - and it didn't have attunements, and the raid progression model was in TBC (and, technically, the same as it is now, you just have a choice as to which difficulty is your aiming - except they've added a permanent swap to a Heroic mode setting).
Personally, I don't care about the sub numbers, as long as there's enough people to do what I want to with in the game. With the increasing presence of cross realm and realm 'mergers,' that doesn't seem like it'll be an issue for a long time. I end my sub when I get bored with the game, and then come back in about three to six months to play for another period of time. It cycles, and I play long enough each time to have fun and see the current level of content.