I agree, but I don't think #3 is possible. For a variety of reasons, both biological and societal, the older people get the less likely they are to have kids, and fewer of them.
That's not something typical outside of that one particular group, nor can you copy-paste that anywhere else.
I think in fact it's the opposite. Because of how universal below-replacement rates are in EVERY developed nation, it shows just how independent it is from factors of culture, economics, environment, etc. It takes a seriously unusual situation like the one in Kiryas Joel for below-replacement to not be the norm.