Another powerful driver of genetic heterogeneity is selection to fit diverse environmental conditions. Our planet earth has a pretty diverse environment and this has shaped a lot of human evolution but some may say that as we increasingly live in artificial environments rather than natural environments which all tend to be rather similar then that may have a small convergence effect but even if true it will be shortlived as humans expand beyond earth because the environmental diversity of earth is a very narrow range compared with the worlds we may inhabit in the future.
Then of course personal genetic tailoring in the near future will allow people to quite dramatically alter genetic and phenotypic traits. Since this will be done on a individual level and might include splicing in even non-human genes we can expect this ability to vastly expand human bio-diversity. There will be as many "races" as there are individuals.
Diversity is also factored by overall population size, the bigger the population then, all else being equal, the more genetic variety. Homo Sapiens could have a serious population reversal but only a apocalyptic catastrophe could really cause that at which point worrying about the homogeneity of homo sapiens (and offshoots) is rather beside concern. In the absence of an apocalypse the human population is virtually guaranteed to continue exponentially growing as our technology opens up new worlds and resources both on and beyond earth.
Genetic isolation is what actually drives homogeneity but that cannot work across the whole species only for small clusters of it and in the long run this cements into speciesation which arguably is a net increase in diversity.
The solution to 1984 is 1973!