Doesn't work that well if people move to other countries to change them to become like their original country. Would only work if travel across borders with the intention of staying was forbidden. Would also incentivize people to change their own country for the better when they can see other successful countries they just have to copy.

> Doesn't work that well if people move to other countries to change them to become like their original country.

You can make the exact same argument against 'states as laboratories'.

That seems like a non sequitur. If each country were left to decide on its own, it seems that many, in fact historically nearly all, would not have open boarders. It seems like you're assuming the freedom of a group to govern themselves would result in them choosing what you personally find ideal. The world as it is now is the null hypothesis. People choose to form countries, and countries choose to have boarders.

If you want a sci-fi description of something like this, consider Too Like the Lightning by Ada Palmer. Enjoyed her take on “hives” which form as groups of people choose how to govern themselves and are relatively free to join. Some choose autocracy, others full democracy.