The Iron Throne should collapse if more than about 4 or 5 kingdoms declare independence. If Robb won independence as the North and pulled the Riverlands out with him, then the Arryns gained independence and the Iron Isles joined Robb to declare independence and take the Westerlands (this is what I did in my game), that should be enough to deprive the Iron Throne of legitimacy and break it up. The Iron Throne should get one last chance to restore itself, and if it doesn't the LPs should be encouraged to forge their own destiny again. It'd be nice to see some alternate titles crop up otherwise, so a King that's been battered that hard could end up ruling an Elective "Kingdom of the South" or became the Blackwater King after all the other LPs abandon them. If a Baratheon presides over the end of the united kingdom, they should proclaim themselves Storm Kings in imitation of the Durrandons. Personally I figure Dorne would be first overboard if the Iron Throne broke up that bad, as they'd probably see no point in supporting the Targaryens anymore (assuming they've completely failed) and go their own way. I can't imagine the Reach would care for much longer if they made up a good half of the Iron Throne's land area and were surrounded by independents.