Lmao
Couple of things that I see popping up:
"Sargeras would be the last boss of WoW"
1) The end of WoW is nowhere in the foreseeable future. EQ1 release date: 1999. Still alive and well in 2013 with 500k subs on the umpteenth expansion.
2) Who says he would have to die. He can make an appearance, we might kill one of his main underlings, and he escapes.
3) This is a fantasy world whose lore expands through an entire universe. Even if Sargeras is defeated, it would be illogical to assume that in all of the universe there isn't a stronger being. He's just the strongest that we currently know of. Hell we don't even know everything about our own real life universe, and this is a fantasy game, anything is possible.
"A legion expansion shows Blizzard is desperate"
1) It's not like this is the wild card they had in their hand the whole time waiting for a clutch moment. This doesn't even have to be the only legion expansion, either. We could have one now, 2 more different expansions and then go back to legion again. There are a lot of notable NPCs still left in their ranks, not to mention the ability for Blizzard to easily weave a few new ones to introduce us to for later down the road.
2) It's not like: "Boom Legion Xpack give us ur moneys", although I won't deny it will probably reel in a few old schoolers and maybe some new people who weren't feeling the MoP theme. I definitely believe that to a broader audience it screamed Kung Fu panda and probably turned a lot more traditional fantasy RPGers away who thought about picking the game up for the first time. A cover with a dreadlord on it I would wager would attract more RPG-subscription-based gamers.