The Burning Legion is disbanded with Sargeras gone.
If Sargeras was still the leader, he would undoubtedly try to stop the Janitor. We already know that the Legion was at war with the forces of Death, since Draka and the spies of Maldraxxus were deployed on a Legion world to spy on Legion troops. So there was war between the Legion and the Shadowlands.
I don't think you understand that the Janitor, in his hubris, seeks to remake reality. It would just be horrible writing if the Light and Void didn't take any drastic measure; especially the Void. It is shown many times that the Void sees Death as the "true enemy", the Void Lords should absolutely send their forces to try and stop the Janitor.
We also know that the Light made a bargain with Death, so that's one more plot thread that remains open.
Would it make for a big clusterfuck of cosmic forces? Yes. Would it also make sense? Yes, otherwise don't introduce 6 Cosmic forces all vying for Cosmic domination.
Since the Janitor seeks to unmake "ALL OF REALITY", it's clearly no longer just Death stuff. It concerns everything and everyone, Naaru and Void Lords included.