Sylvanas clearly isn't pleased with them, and they formed without her consent. So why doesn't she just kill them off? I thought the "government" in Undercity was pretty much "You do what Sylvanas says or you die". So isn't this "Desolate Council" a clear usurpation of power? Basically a coup d'etat? What happened? Did a couple of random Forsaken come together and go "Well, Sylvanas hasn't been here for a couple of weeks now, let's take over as rulers of this city"?
I'm honestly surprised she isn't acting on this. You might say "But the people of Undercity would not condone her killing the Desolate Council", but why not? I believe the rules are very clearly explained when a new Forsaken is created. You either follow Sylvanas, or you fuck off. And if you try anything to hinder the Forsaken in any way, you die.