Which is funny since we beat him there on Mythic. Guess he just ports everyone back at the last second to make sure he doesn't die.
Laziness I assume. Either way iirc there was a tweet a while back saying he was dead for realzies on mythic so I'm not that concerned about it.
Gul'dan is totally going to come back though. I'm sure he will be a part of whatever they come up with as a way to get the Burning Legion to invade MU Azeroth. Which, meh, whatever I guess. I don't think he will really be a key part of the story when it happens anyway, will probably just be the plot device that sets things in motion.