It need not be theoretical connections to the ancient past. Consider a people who's beliefs get further and further from reality and who's society can't manage

It need not be theoretical connections to the ancient past. Consider a people who's beliefs get further and further from reality and who's society can't manage to maintain, let alone build, what it used to like roads and bridges. Such a place is in deep trouble almost by definition. And even more oddly, not due to lead pipes or barbarians but purely from social-political causes. And if we're talking about the US, the world will continue on with it as a 3rd tier power just as it did outside of Rome. Either the Europeans will get their act together or China will dominate with a capital centered dictatorship as the standard.