>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.

You have no control over who comes in when you build a system that requires like minded people.

It makes it just as easy for people completely against those like minds to come in, in any number, and vote with equal voice.

If you have a successful nation/economy, people will want to come whether like minded or not.

>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.

The Cartels would take over the the southern united states in a hurry. It would turn the "border" into a series of criminal empires even beyond what exists now. Asian and other gangs would experience massive surges as they import members from overseas.

Insane policies that have no connection to reality and assume there are no bad people.

>no help for the poor since that would attract enough free riders to kill the goose.

while I agree that you cannot incentivize "free riders", I don't think your system is viable with open borders either way.

You will be conquered and you will have no national unity because everyone will bring with them their own ideals and concepts, regardless of your belief that only people who agree with you will come.

And it will be a massive betrayal of your citizens to give away their future to anyone who walks in, all in a massive self sabotaging effort of malignant empathy.