The portal is destroyed.
Thrall kills Garrosh...
Why are we still here? why is this expansion still limping on?
Ignore alternate universes and time travel. our job is done. We destroyed the portal and kill Garrosh. We can smash our own portal behind us and cut them off?
Gul'dan and the Legion is happening in an alternate universe in the past, if they haven't been a threat to us before, they won't be now. They would of already been so by now. Our universe is 30 odd years ahead of theirs, yet they haven't done so (if we use Garrosh's magic crystal as a guide).
I know ignorance is bliss and I know I am really obtuse, but we have already defended and secured our own realities safety. Why linger?