There is a well researched psychology of doomsdayers. Famous papers.
I'd have to look up the cite (it's in the recent Chris Mooney book). But the psychologists joined one group, when Doomsday didn't happen as planned, it did not shake their faith.
Whilst the assumption that history is cyclical and we go through periods of things getting better and worse (ask anyone alive 1929-1949 about that!) is valid, the assumption that we are living in the last days of the Roman Empire is usually not. And, even then, with the exception of some cataclysmic events (volcanic eruption and tsunami, plague, civil war, barbarian invasion) most of the time the decline would have been less perceptible, just a gentle wearing out of things-- roads and bridges not being repaired, people moving to the city for more protection, fortifications being built around towns and farms, etc.
Think of modern Detroit you get the picture. The place has been dying for a long time, it's only in the last 10-20 years or so, perhaps,that it's demise has become obvious.