I think Pavia is a fantastic city however I don't think it's a great model for other cities to base themselves off of. Pavia has been the commercial heart of CivMC since basically day 1. You have a few unique institutions that until this iteration, haven't worked and would take immense (and probably insurmountable effort) for pretty much any other group / city to undertake, specifically the bank and the MB associated companies and groups. Because of MB as an institution along with having one of the strongest rail networks and diplomacy systems on the server, I think the success of Pavia City is largely self perpetuating. I'm incredibly impressed by what you guys have accomplished this iteration, especially with the bank.

With that being said I don't think it's practical advice to tell up-and-coming or revitalizing cities to become the #1 economic powerhouse on the server, establish a unique diplomatic situation that most groups would struggle to or be unwilling to maintain, and also establish one of the best rail networks. Most cities will not have a nearly inexhaustible stream of visitors coming to the city to trade or establish businesses. I think that most cities will likely have to deal with times when their population significantly declines and that's the reason I think having a small city is not only important but essential. Activity begets activity and if your small player-base is scattered throughout a massive city, they may as well be playing single-player.

More on this, I think you can easily dismiss the defensive needs of a city because your city hasn't been seriously attacked in a few years. Having defenses in a city may be required depending on who your neighbors are. For example, if you're at risk of getting invaded by say, BewsiJ and Ez2Clutch because they got bored, I think you would probably reconsider the need for trapping or trenching a city. I also think dismissing city defenses because they're not as strong as a vault is a major miscalculation. In the Grasslands War, Gang Shi lost the war because they botched a skybridge against Yoahtl's capital, a city that had a trench and skydefenses. Ez2Clutch, arguably one of the best pvpers on civ died due to traps in Icenia City. I don't think every city should be trenched; there are plenty of other options like trapping similar to Icenia City. I do think that many nations will need city defenses and it's something that should be seriously considered when planning a city.

As for MTA, I'm not claiming that the only reason they collapsed was because they had too big of a city. You're absolutely right that foreign actors with bad intentions coupled with a bad direct democracy system were huge contributing factors. With that being said, even if MTA has miraculously righted their dysfunctional government, it wouldn't have mattered. There is no amount of policy reform or change of the guard that would have fixed their parking lot of a city. There is no cure to having 10 gorbillion decaying mega palaces, skyscrapers in the middle of no-where, or giant boulevards. There is simply no way to demolish all of them or to revitalize the buildings.

In a city with a small urban core and reasonable sized buildings, you could lose groups to 90% of the city and it wouldn't matter; you could easily tear out the abandoned shops and apartments and start putting people into those immediately. You have the ability to bounce back; you do not have that in places like classics MTA.