I've been thinking of this for a while. Sargeras is probably the ultimate boss for many people who have been playing WoW for the past many years. It would make sense for the team to finally bring this story to an end, providing closure to long-time dedicated fans and to the writers themselves.
The expansion even bears the title. Legion. If this isn't the final Legion expansion, then what would you call the one that is? They went all out on this. Sargeras may not die in Legion, but we will stop them once and for all, that's for sure. They are introducing Azeroth as a titan, letting players wield legendary weapons from lore (Ashbringer being the most obvious), the elves are finally banding together, Gul'dan will die. Where will we go from here? There is no doubt that this expansion will finally wrap up a ton of threads that are getting tedious and boring. They're preparing us all for the ending we deserve.
Now, I'm not saying WoW will end, of course not. But they're ready to take us on a new journey. I think that the new lore about the Void Gods and so on is evidence of that.