I agree with you that immigration has generally been a good thing and that the vast majority of immigrants would not pose any problem. However, I'm not sure th

I agree with you that immigration has generally been a good thing and that the vast majority of immigrants would not pose any problem. However, I'm not sure that it's fair to just ignore the fact that open borders could have serious complications that would need to be addressed. If a large number of impoverished, unhealthy, or uneducated people moved to a country it could overwhelm the resources available to help them. Increased population could overwhelm the availability of things like good jobs and housing. Open borders may not be reciprocated. Many people could move to the country for economic reasons but be uninterested in or unwilling to become part of the country's culture. Many people could move to the country that do not value norms such as democracy, freedom, equality, etc. There is a limit to how quickly society can adapt to cultural change. If too many people with a dramatically different culture moved to a country, it could cause unrest.