Why are you so optimistic? I can't think of a single example of two ethnically distinct / reproductively isolated but geographically intermixed groups that merged after centuries of staying isolated.

Even places like Brazil, where race is more of the sort of spectrum one would hope would convolve away, have plenty of issues that aren't abating.

Are you saying that after 100 years of mind bogglingly fast improvement on this issue (in terms of the history of humanity) we've stopped improving? Hardly.