Earlier quoted context omitted.
Social safety nets, open borders, future ongoing existence of an orderly nation. Pick two.
If you think open borders and ongoing orderly nationare an option you're wrong. Whoever walks into your nation will shape it. Your children's voices don't matter. There are far more children outside the country than in it and democracy can't work with open borders.
I think that a nation could in theory have open borders and prosper, but the trade-offs would have to be such that only like-minded people who can fit in and succeed would have an incentive to immigrate. This means that their children's voices would be saying the same thing as your children's, but there would be e.g. no help for the poor since that would attract enough free riders to kill the goose.
So I agree with you that borders are preferable, but I don't think getting rid of them is totally impossible, but I do think that doing so would be even worse for the compassion that makes people want to in the first place.