A clearer view might be, the population distribution changed. Fewer 'purebred' anything existed over time, and more mixed individuals. Until today we have a lar

A clearer view might be, the population distribution changed. Fewer 'purebred' anything existed over time, and more mixed individuals. Until today we have a large fraction of humanity with both genes. It's not necessary to posit a catastrophic change. There were simply more of the new kind, so there were more of their genes in the mix and fewer Neanderthal.