How certainly do we actually know that? If the evidence you claim is that there are no Y-chromosomes from other hominids, an alternative explanation could be that hybrids with Y-chromosomes from other hominids did exist, but disappeared gradually over the centuries/millenia either because they were less fit or through pure chance.

Are there any now living people with mt-dna from other hominids?

Having a minority non-autosomal genotype could be an issue as there might be recurring "compatibility problems", given that the autosomal dna would mainly evolve to work with the majority mtdna and ydna.

The genetic evidence points into the other direction:

> There is considerably less Neanderthal ancestry on the X-chromosome compared to the autosomal chromosomes, which similarly suggests that admixture with modern humans was primarily the result of mating between modern human females and Neanderthal males.

https://en.wikipedia.org/wiki/Neanderthal_genetics#Parentage

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