Did Rome ever fall in the first place? I've been there. If it did fall, it didn't hit the ground too hard. Just saying...
It wasn't often that cities were completely destroyed. They weren't usually sacked on orders from a general; they were sacked because the general couldn't be bothered to control his soldiers.
Most countries that the Mongols invaded had half their population killed, from China to Poland; eight million people left Ireland in the 1500s-1800s, more than double the country's population; a third of the population of Germany was killed in the Thirty Year's War. They are some of the richest countries in the world.
I think this would be the perfect time for the formation of a Republic... each Province would send representatives to the (newly renamed) Republican City who would be the lower house (the upper house being the Elder Council). The combined houses would legislate as well as elect the primary leadership position (Prime Minister) and appoint military commanders.
There's no record of that in Imperial culture; the Nords seem to have considered something similar to it. The result was a civil war and the destruction of their empire when one of the nobles wasn't happy with their decision.
The Argonians were never actually conquered, they just agreed to pay some taxes to end the war. They could re-make Argonia at any time
This also applies to Morrowind, and possibly Hammerfell.