They always get a small spike on expansions because people will resub for the new content, play for a few months, then promptly quit again. There's no longevity anymore. In the past, just getting to max level could take someone a month, then a few weeks just to gear for raiding, then months in the first raid tier. Now, the entire game itself is cleared and done in less time than we used to spend just getting to max level. You'd think more levels would take longer, and it should, but it actually takes far less to get all the levels now, than it did in the past. It has gotten worse every expansion, because with each one comes another XP cut from 1-max, on top of all the talent tree buffs, dungeon nerfs, heirlooms, and so on.