I took "join the community" to mean "this rule applies to would-be immigrants", in a way that forces them and their kids to assimilate. The practical value would be to keep that minimum of incoming gene flow to stave off long-term inbreeding in your hypothetical city-state.

Thing is, you want to keep it small, then you can't have "endless population growth", or you'll just wind up with the same stupid problems. Not everyone needs to breed, but breeding can be encouraged without totalitarianist bullshit when needs be (Baby Bonus was a nice incentive.)

I took "join the community" to mean "this rule applies to would-be immigrants", in a way that forces them and their kids to assimilate. The practical value would be to keep that minimum of incoming gene flow to stave off long-term inbreeding in your hypothetical city-state.

Thing is, you want to keep it small, then you can't have "endless population growth", or you'll just wind up with the same stupid problems.