Victor Davis Hanson, a well-known historian and scholar whose books are on my book list, has just written an article entitled Why Did Rome Fall—And Why Does It Matter Now? that is well worth reading and pondering.
Hanson argues that affluence sapped the hard-working spirit of the Romans, but that their system continued for a long time because it kept encompassing new peoples from the periphery whose spirits weren't yet sapped by affluence. He wonders if there isn't a parallel in American life, in which the hard-working, thrifty habits of our forefathers have largely dissipated, yet we stay afloat because of all the "new peoples" (read Chinese workers, these days) we keep encompassing in our global economy - people who are still willing to work hard.