> Of course this niche doesn’t exist everywhere

Iceland has extensive support for disabled people though, irrespective of abortion policies. These are of course not mutually exclusive things.

It doesn’t really help if you’re trying to branch out the hominid tree into a future Homo mongolensis, but don’t even get to see the light of day to enjoy any of those accommodations.

I suspect that we would only see a speciation event like this if people with Down's syndrome were segregated from people without Down's syndrome and allowed/forced to form a parallel society. It's hard to imagine modern human society tolerating such an apartheid, leper-colony style of treatment of people with Down's syndrome, however.

Segregation would take them out of the ecological niche where normal people provide for them. They’d be back to Stone Age survival outcomes.

Besides there is already a social taboo against engaging in sexual intercourse with them because of the substantial power asymmetry. So really the only impediment to speciation is the lack of fertility.