While immigration can solve the literal numbers problem, if there is no successful integration -new citizens share the main values of previous citizens-  (and the bigger the immigration needed, the bigger the challenge), then you’d be replacing one set of values for another, not solving for why a specific set of values leads to inevitable lower birth rates 

 The objective version of that is that you're replacing most of a less-successful culture with most of a more-successful one, but the former has a better chance at partial-extension. 

 While tend to agree (just like a gene, a natuon that doesn’t reproduce can’t be easily categorised as succcessful), let’s not forget many of the less successful nations have higher birth rates (a steelman arg is higher birth rates are a response *against* a declining nation, but still ) 

 It's not pure birth rates, as a success measure, but rather, the number of fertile offspring that survive long enough to produce their own offspring.