That doesn’t work though, over 1 generation the TFR and family completedness of 2ng generation immigrants coverges to 80-90% of the native one and over 2 it seems to be on par or lower.
This is without all the social issues of putting them in ghettos and pulling up the ladders to their kids to integrate in society.
It works like debt, in the sense that you are taking a band-aid solution and kicking the can 15 years but they have been doing it since the 1980s so the can is here now.
So you just keep adding new folks? That's my point, we need immigration, systematically, or our societies are dead.
Ok and people are blank pages without cultures or tendencies, which means we can just graft them without issues?
There is also a dilution process by which you assimilate or adapt a population and if the % of new people is too high the culture that produced those societies basically disappears.
You need integration initiatives and strong enforcement. Integration is a two-party process, if either side doesn't take it seriously, it won't happen. We should be generous in giving out chances, but strict in taking them away from people that prove they're not making use of them.
The last argument is empirically incorrect. Think of Vienna for example, most of the population there has roots from all over the Austrian Empire. Yet, it's distinctly Austrian.
It's not a given, it takes effort. But it can be done.