As the game ages and more people become more familiar with its content I'm sure the dips would become more dramatic, as less people would want to play after exploring whats new, but the overall trends would probably be similar.
I also wonder if the content itself is getting a little less inspired and focused as the senior folks move on to the new MMO. I definitely get the impression that B has put all of their new-IP energy into that one basket, making a culture already geared towards retreading decade-old ideas that much worse.
I'm sure you'd even see similar patterns for content patches.
I watch that chart, and don't recall anything like the horde year-on-year dip in the past three or four years. I'm just wondering if the title has peaked in terms of market saturation - seems like I see an ad for wow on every third webpage. "Hey, granny, you're the only one left in the world that hasn't played WoW for a few years and maxed out at least two toons! NOW IT'S YOUR TURN..."
Only place i can think of is (yet another) demon expansion where we're going to explore the Draenei home planet (home to the Eredar now) Argus and even the Legion home planet. Return of Illidan (we're used to revivals) and to kill Kil'jaeden and Sargeras. Then it most likely is going to be "the end".
'Back to space' is probably a good guess. It won't be the end - they'll still do XPs at least until the next MMO ships. Ten is a nice round number.