I'm sure someone has studied why all those empires eventually failed. Is there a common thread? What is it? If we knew, we could avoid it.
The same reason most family run businesses fail after a few generations. The first generation builds the business, the second generation holds on but without putting in much effort, and the third generation tends to squander the wealth with no appreciation for what it took to gain the wealth in the first place.
That mindset runs throughout this country, from the ghetto to the suburbs to the mansions. By and large the populace of this country has no appreciation for the blood, sweat and tears that made this country great and they are willing to squander that greatness.