Yes. And even the Lich King has some importance in Legion.
I see Legion as the end of an age. Warcraft story is being wrapped up so it can start anew. Characters are dying, open story threads are being closed, there's a chance of a new generation of leaders (and maybe even player characters).
Whatever next expansion is about, I bet it will: develop the Horde, deal with rebuilding the world (or getting the resources to rebuild it) and seed new villains/plots. I don't think the Void is going to be the main threat any time soon, it will probably be developed for many years, appearing as side quests, until it is ready to become the center of an expansion.