Transience is an enemy
Transience is often the enemy of community.
If you have a city of migrants you often have people completely disconnected from everyday politics. I lived in London(for work, not university) where I had a lot of friends, but no reason to go back because no one i knew lives there anymore.
London is the epitome of this. Many people have no idea what constituency they are currently living in and have zero interest in community projects or planning arguments because they never intend to be in the same place in 5 years time.
Without an "indigenous" population with roots (mainly family) to appeal to, I dont know how you can appeal to people to set down roots, join police forces, become teachers or politicians etc
Cities have been many things in the past, some of the very best and some of the very worst of human creations but if nothing else they are the melting pots where people come together and learn to understand and live with the different cultures, worldviews and experiences that exist across our culture…
Possibly our best hope of a future could come from our cities having more influence over the way that thoughts are thought and deeds are done with perhaps the ultimate aim that Mankind can move more and more away from the rural realms and enable us to start to hand these areas back to nature.
I think perhaps we are going to have to be prepared to think of doing something like this if we really are going to find ways to continue existing in anything like our current numbers, in relative peace and equanimity on this Planet which we are rapidly changing.