One thing open borders would completely destroy would be the social safety net of many countries. Let’s say there is completely open borders between Sweden and the United States. Sweden has high taxes and an significant social safety net. The United States has low taxes and a non-existent safety net. Over time, people who pay a lot of taxes migrate from Sweden to the United States. People from the United States who are unable to work for whatever reason, migrate to Sweden.
This would also affect government. You either end up with a large underclass that doesn’t have voting rights (see Qatar for example) or else they have voting rights and the laws of the receiving country change to reflect the culture of the immigrants.
Taxes would likely change from income taxes to consumption and property taxes, so it’s unlikely that governments will go bankrupt. More attractive areas of the world will cost more to live in, but it would eventually turn into one large country with pockets of exclusive communities.
This would also affect government. You either end up with a large underclass that doesn’t have voting rights (see Qatar for example) or else they have voting rights and the laws of the receiving country change to reflect the culture of the immigrants.