That is a very good point, but it only works of the world is somewhat open to immigrants. I migrated 3 times in my life, starting from practically one suitcase each time. I'm very lucky to have ended up in times and countries which allowed me to stay and to contribute to their society. Sadly that seems less and less a given.
Yes indeed, and it might still be OK, but the world moving to the right and the right being so against immigration I forse a future where permanent residency is not that easy anymore. Especially if you're not like me a white fairly well educated male from Europe. People like me seem to have huge privileges in many the parts of the world.