[_nalply]

I think the possiblities that a minority can leave the group is an important escape valve for them. Now assume that in the future they are going to be not such a minority anymore like today, then what happens? Maybe some oscillation like for Lotka-Volterra situation, perhaps a bit different?

[skissane]

The possibility of defection to mainstream society actually increases the selective pressure against defection-promoting alleles/memes, compared to if that possibility didn’t exist; the mainstream acts as a genetic/memetic sink to absorb those alleles/memes, reducing their prevalence in the next generation.

I think the fact that in e.g. the United States there are multiple such high fertility traditionalist religious minority groups coexisting makes it unlikely any one of them will end up completely dominant in coming centuries. More likely, I think, conservative religious groups will ally together across confessional boundaries to attain shared policy objectives (e.g. government funding of private religious schools). As such, I don’t think the possibility of defection to secular society will disappear; secular people will still exist, just numerically and politically weakened compared to today.