Yeah that's the point there aren't any examples of multi-ethnic multi-cultural cities that work _without_ some kind of authoritarian rule or a common culture/language that all are expected to belong to.
Are you suggesting that it never works without coercion that only full assimilation, the eradication and rejection of imported values, works? This is simply not the case as the development of almost every major European and Asian city demonstrates.
Your idea that cultural dominance is the driving force only works to a point in attracting capital, a limited amount. People are different with them come ideas, even when they hate each other the exchange of ideas happens it is what drives commerce. Money and ideas are what changes a trading post into a significant city.
The great belt of cities along the old silk route was dominated by Turkish people using Persian ideas. European cities only became world beating once they started banking and trading outside Europe on a large scale. Russian cities became stone cities after the Greeks showed them how.
The driving force for all this and more was an exchange of ideas and people not dominance, it was an internal readjustment forced by external pressure that made wealth creation possible.
People follow the money that’s why market traders in central Asia are mostly of Turkish origin as one example, the various ethnic quarters in all cities reflect the cultural exchange and development that migration brings and is necessary for growth. Ireland was the only place in Europe to expel the Vikings and they had to be the only Europeans to invite them back. Spain expeled the Jews and had to import Muslims to do the irrigation work.
Maintaining an identity was what made this growth work, both then evolved together in a slightly different way. People opted for this as the community gives protection until individuals are confident enough to leave the group it’s capital that gives people acceptance who wants a poor person in the golf club?
Imposing restrictions upon people ultimately stops the flow of capital the modern equivalent is economic sanctions. The Chinese discovered this when they turned their backs on the world.
Problems with mass migration today are more about class than anything else as it was in the past with the fear of the mob.