actually, one of the main reasons was basically there was no incentive to work, no reward, so small communities started doing for themselves, and the whole suffered...
all empires or countries has its up's and down's like this http://www.bom.gov.au/weather/radar/about/images/wavelength.gif The chinese always traced their dynasties and even saw this kind of result. i don't know about a collapse, but eventually there will be a down period or a dark period. how long it lasts or how deep it gets, i don't think anybody can really say until it happens.