Rural conservatives who have kids will eventually have those kids leave for good paying jobs in urban areas, or those kids will likely not have a great life expectancy from lack of opportunity in their small community (dying small communities are well documented: https://www.google.com/search?q=dying+small+towns) and deaths of despair (alcoholism, meth, opioids: https://www.buzzfeednews.com/article/carolinekee/maps-despai...). As long as prospects for women continue to increase (education and earning potential), and the data shows well off educated women prefer a partner on equal footing, conservative birth rates will eventually converge with that of other ideologies (this can be inferred from the total fertility rate trend for religious vs non religious women of childbearing age).

While your point about ideology drift over time has some merit, I don’t believe it to be significant enough to materially consider.

https://www.journals.uchicago.edu/doi/abs/10.1086/706889?jou... (https://doi.org/10.1086/706889 if you want to grab the full paper from SciHub)

“Consistent with previous research but contrary to folk wisdom, our results indicate that political attitudes are remarkably stable over the long term. In contrast to previous research, however, we also find support for folk wisdom: on those occasions when political attitudes do shift across the life span, liberals are more likely to become conservatives than conservatives are to become liberals, suggesting that folk wisdom has some empirical basis even as it overstates the degree of change.”