Since routes are not automatically closed, however are frozen, when you leave an alliance it is technically possible for a new entrant to break the caps. As i recall, many of these routes become un-editable until rectified (but i forget exactly if it works in practice).
For the most part, alliances are designed to be great for all parties involved, however there are a few grains of sand thrown in (by design or by chance) that are supposed to keep alliances relatively reasonable in size. Only way to do that without forcing conflict is to throw just enough of a chance of conflict inciting situations as is reasonable to keep the mega-alliances on their toes, without disrupting small alliances as much.
