Chalid said:
Simply Imagine someone would have decided that rome would no longer be a city but will become an outpost and all people have to immigrate to a former "settlement" of rome eg. a city in greece. Do you think that would have worked? i don't think so.
Bolghar -> Bilär
Potsdam -> Berlin
Rome -> Ravenna -> Rome
You'll find several cases where a large city was degraded to a lesser size because someone decided that the capital (or other important functions) would be transfered elsewhere.
Many towns do survive this, though loose a lot of population. Others over time even simply cease to exist, but thats very uncommon.
Especially your example Roma is a good one. When the Goths sacked it, it was replaced as capital with Ravenna. Rome nearly ceased to exist, only due to the function of Rome as center of the catholic church it didn't.
Yes it would be untypical to have this kind of "selective devolution" as we might see with the Kuriotates.
Though on the other hand, the continuous city growth as seen in vanilla civ is unrealistic as well. If you look into european history you'll see how certain towns became dominant for a few centuries, but fall back into obscurity (some even loosing town status and becomming villages) later on.
Many important towns of the german middle age (Nürnberg, Aachen, Würzburg, Lübeck) are only 2nd or 3rd line cities today. Out of the three major cities of Germany (Berlin, Hamburg, München) only Hamburg was a major city prior to the 18th century.
While Lübeck (the capital of the Hanse) for example hasn't become a settlement, it has only little more then 5% of Berlins inhabitants today.
So actually yes, it did kind of work.