If there are no good options to migrate to, that doesn't provide a way out. And, as a migrant, that's ignoring the cost of migration (e.g. the decades it takes to lose a speech accent and not be seen as "the foreigner", assuming your ethnicity allows for that anyway)

I'm not at all sure if voting to splice a country should be a thing you can do, maybe there is more merit in some international right to a proportional vote on something (no FPTP system) so you get better representation of all opinions, but simply saying "you can always go somewhere else" seems a bit too simple

It’s also a very anti-democratic option whether taken voluntarily or forced.

When you leave you cut all important ties to the polity, you surrender your ability to participate in democratic processes and you revoke control over yourself and your property. It’s the nearest thing to direct violence you can do without crossing that line

It's actually not available to most people that live in any third world country, otherwise migration would be significantly higher than it already is.

Regardless, "voting with your feet" is an individual action. Voting at home is a collective one, representing the will of not just you but the people from the place that you come from and were born into. Only one of those reflects the ideals of democracy, if that's really the ideal being strived for

How does one draw new political borders that way? There is practically zero unclaimed land.