You can't really have completely open borders until you establish relatively equal levels of economic and social well-being all across the world. That can't be done in a purely competitive system based on self-interest.

It's not going to happen in our lifetimes. It may happen in some distant future. If you believe in reincarnation, then cross your fingers and hope it does. If not, resign yourself to the fact that it ain't gonna happen in the world as we know it.