I realise that anyone who agrees with the core premise of Kaufmann’s book will disagree with me on this.
Firstly, I think that deindustrialization will protect us from literally going extinct from low fertility. If the number of humans ever falls to the thousands (say) we simply won’t be able to maintain a complex society and life—including fertility habits—will look similar to ancient times. Population will begin to rise again.
I’d consider the Amish to still be at the “small, high-fertility subgroup” stage, although at population 380,000 they are arguably becoming an exception to my rule. For a group midway through the “regression to the mean” transition, we could maybe look at Mormons. In the early days of the church, polygamy was encouraged and large broods of kids were celebrated. Apparently Brigham Young and another early LDS leader each had over 50 children. High fertility continued after polygamy was abandoned, but has fallen a lot in recent decades. In a 2011 survey Mormons “report having had 2.6 children on average, compared with 1.8 among the general population”. Other estimates put the Mormon fertility rate at 3.4.
I agree that it’s possible that some subgroup will eventually buck the trend and form the bulk of the population. 50 years ago you may have wondered if this could happen with Islam—but fertility in Muslim countries has tumbled since then. I would expect that a “high-fertility takeover” is far more likely after a catastrophe, when there are fewer people around, and survivors are more susceptible to new beliefs and in need of new affiliations.
Only if you consider artificial people to be fundamentally less valuable than real people. I’m reserving judgement on that until I meet an artificial person.