I am pretty sure that even when WoW ends, we will not be killing sargeras. The warcraft brand is too expensive for the "master evil" to get killed. Plus if it were true, we would probably kill him at the end of the game's life when a lot less people are playing. They probably wont destroy their (half a billion dollar worth?) brand for that small amount of people. It is much more profitable to have a WoW 2 in 10+ years from now. Just from name recognition alone it will sell huge amounts.
Maybe we will kill all the old gods and in some way still get to keep the earth by titan intervention. Then WoW 2 or an RTS can start a 1000 years from now where there has been peace across azeroth, history has become legend yadayadayada untill one goblin finds a helmet on top of what looks like a man made mountain in a forgotten frozen continent up north. And at the same time a rift has appeared in the eastern kingdoms, voice appear, they whisper the word Sargeras which nobody can understand.....DUN DUN DUN.
Anyways, a new era of Azeroth with new lore. Something like that. They do need to go out with a bang though so perhaps the old gods will get killed.