The same way you would with a bigger population for any reason. There’s an added bonus that if rights erode, people can simply leave to somewhere less oppressive. People can’t leave oppressive places due to borders. Less oppressive places may become oppressive but that would drive people elsewhere.

I’d use the US as an example. In a real sense the US is a collection of small countries with a federal system and free right of migration. For this discussion the federal system is irrelevant. How does California or New York or other high populace states ensure erosion of rights? I’d posit part of the very reason they’re so attractive is they tend to hold rights very high on their list of priorities. More oppressive states, likewise lose populace to migration because unless you’re in the in group you don’t belong there.

Cultural erosion is IMO nativism and bias. Time is change, no culture is static, and even our oldest cultures are young compared to humanity and look nothing like they did 100 years ago. Changing populace doesn’t change a culture any more than time does, it’s just easier to blame someone that doesn’t look or talk like you for it.

Your examples of mainland Chinese citizens or Americans moving around are only relevant because the issue of borders to begin with. If there were permeable borders we would be in a steady state and you wouldn’t see behaviors like that. Maybe in the beginning you would see artifacts of history manifest weird transient behaviors, as you do in any suddenly changed dynamical system. But in steady state your scenarios would be as likely as all democrats moving to Georgia to change the election. It sounds good but in practice didn’t happen.