Civil war or a succession conflict is a likely catalyst for the eventual decline of the Argead Empire. Philip III's reign will be the zenith of the early Argead Empire, but eventually things will go wrong and the empire will be on the defensive. I'm pretty sure I've already hinted in one of the updates about where things go awry for the Argeads.That certainly was a swift fall.
I'd think that if the Argead realm does fall, it will do so to a civil war. At this point in time, an outside threat being absolutely devastating would require something like time-displaced Mongols arriving. Unless they grow really decadent?