So yeah, we infiltrated their home world and imprisoned their leader.
The same thing happened with the Scourge, with Scourge armies still being numerous enough to sweep Accross Azeroth.
That said, Illidan did not do a Bolvar and did not become leader of all demons, so I assume they would still continue to be a threat to us, after all the Titans were not able to completely rid the galaxy of them, how could we?
By their accounts, they still control most of the galaxy, having burned or infested countless worlds.
Are we to expect more legion invasions, or do you think they are gone for good?