Will everyone in the world eventually become the same, mixed race?
It depends on what "eventually" and "mixed" means. If "eventually" means in a thousand years then evolution will do its thing and people will more and more intermingle, but it will take a long time. If "mixed" means 50-50 then this will also take a long time, if ever. But if "mixed" means, say <1%, of another race, then this has happened already for most people in the US already!