I wouldnt be so hasty to take them outside the picture overall.
Sargeras' burning legion is gone but demons still are a formidable force and always attracted by arcane power. And we know that the surface of the planet will be flooding with it in the near future.
And although they lack leadership we need to be fair : The titans had their 2 greatest warriors trying to keep them under control and not some random titanforged/celestials or w/e. And on top of that the whole bunch of titans and the most iconic demonhunter are out of the game for the time being.
So although not a large scale invasion demons could still make formidable foes for 5 man content or small raid tiers.
Not to mention that KJ was killed suspiciously close to Argus and I don't know what planet we could see in the background during Archi mythic encounter (could be Argus or w/e dunno if something about it is confirmed or easily retconned).