Hm. Which ones would that be? Cities like London, Paris, Cologne, Marseille, Barcelona, Carthago, Cherchell, Carnuntum had that for at least three centuries, which I'd count as well-established, but there were Dark Ages at all places. These regions didn't just choose neo-barbarism on a whim, they were overrun by Germanic tribes who first pillaged then took control. But then in most places, in spite of the population loss, the existing majority populations with well-established settled life culturally assimilated their rulers (see the success of Church Latin, Athanasian Christianity, Romance local languages in France and Spain).

Italy had been so destroyed by the Roman slave-economy

What do you mean? Wasn't there slave economy on all parts of the empire?

_{*Lunatic*, n.
One whose delusions are out of fashion.}