The internet is probably what keeps an empire from crumbling for let's say ridiculous reasons. The fact that we're a global civilization gives you all kinds of protection against all kinds of things. It would seem to me that any empire would be a vulnerable to all matter of you know, risks. So sooner or later, and maybe it takes about 250 years, something gets you. But I don't think there's necessarily one obvious reason why any empire would only last 250 years.