The end point of Legion was likely established early on: actually beating them on Argus and imprisoning Sargeras, so they needed to get KJ out of the way for that to happen
Does feel wasted though, and the mythic encounter itself was just really frustrating with the long intermissions and the pass/fail, and hoping for good spawns in the Illidan phase since you really needed to kill everything in one go. Spaceship was cool though, but fighting him in the Nether was far too convenient.
Sunwell KJ was still epic for its time, lots of cool abilities, though much easier than M'uru. Input lag was a killer though.