Say that Alexander dies of an illness or accident at around the same time as Philip, and without him the Macedonian hegemony collapses, the Greeks fall back into fighting each other, and there's no Hellenic state left which could launch a major invasion of the Persian Empire. How do the Achaemenids fare in this scenario? Would they end up falling anyway due to internal strife; would they manage to set things back on track; or would they keep hold of their core territories but lose some of their more distant provinces (in Asia Minor and Egypt, for example), whether de facto or de jure.