Not sure it was so empty. The Maquis were one thing but even Eddington noted that it was almost unthinkable that people would leave the Federation. So, the idea, when the Federation had just survived through nasty wars, losses and substantial cost of resources and lives, and then have people go "Hey, we're going to quit" probably hit quite hard to the Federation as a whole.
And where would they go? Well, 17 planets is no small segment. So, they could easily align themselves together to negotiate with other regional powers.
I think there are a lot of factors at play, so to say its an empty threat is, well, a bit empty, to my view.