Fertility is very well studied and the causes of low/high fertility are well known throughout the modern era. Sub-replacement is a very modern concept, coming with the normalization of contraception and dual income households, not with moving from polygamy to monogamy.
Likewise you keep stating elsewhere some "modern notion" of infidelity. There is nothing modern about it. Men came under the same classification with the spread of Christianity through Europe and those societies didn't all crumble and wither away. Even among the elite, the bastard children had very significant challenges in their rights and were not seen as coequals to their peers borne from the monogamous unions.
I don't know if you are an LLM or a bot, but even a cursory look at history would really help, instead of your flat-earther like approach throughout these threads.