I imagine there is a balance if the countries both have people-friendly policies and it is just the development status that differs. People choose where to live on quality of life, not income; income is only one factor. Lower costs of living work in favour of people staying / moving to that country, but low incomes work the other way.

I imagine in low average quality of life countries people benefit from mobility (companies there are competing with not just local companies, but the option of moving somewhere else), but local companies have to reduce their profits. In high average quality of life countries, high mobility negatively impacts local people (they are competing in a race to the bottom with people from other countries) and helps local companies (they can lower wages). Since there are winners and losers either way, I’d personally rather policy globally be optimised for people over companies, and helping have nots over helping haves - which means opening borders to help the people in lower average quality of life countries.