Why would you assume the population cap is arbitrary? There's a calculable limit to the population an area of land can sustain. (Yes, some agricultural practices can mitigate that, but that should also be weighed against culture and history, and how much change is acceptable.)

https://en.wikipedia.org/wiki/Carrying_capacity

As far as I understand, action begins when the population hits 9.5M, so likely no-one gets kicked out, but fewer new visas will be approved, etc.