I'm looking at some videos about city population and it seems that Rome just falls of the map, from a million people to under 200,000. Why did this happen and why was it so seemingly permanent as compared to cities like Constantinople? What was the the reaction to this from those ruling the city?
/u/Sunagainstgold wrote a great response about Rome's population post-fall: not a direct response to your question but very related: https://www.reddit.com/r/AskHistorians/comments/7uweo1/the_city_of_rome_had_a_population_of_over_one/
Definitely don't want to discourage further exploration of this topic, but /u/Tiako's earlier coverage of this topic is well-worth reading, as is the ensuing followup discussion.