If you are referring to the US in your unsupported decline assertion the numbers don't support what you are saying (I disagree the US is more in decline than it was in the 70s/80s. It has different structural problems today, like housing and wealth concentration, but that isn't the same thing).

There's much stronger relationships to religiosity and fertility rates (with a much larger than income based gaps), regional/cultural choices and fertility rates, than income. India, which we are discussion here, supposedly a country where the quality of life is rising, has surprisingly low fertility rates.

IN your example it's much more likely Bob is no longer religious, Bob has moved to an area (or a culture has set in) where having less children is the norm/social structure. Among my social group having a child was very much 'catching' with friends having clusters of children around the same time. A culture of not having children would create the same opposite effect. Instead of talk about coming babies, shared excitement, feeling left out/un-adult, surrounded by hormones, if you have a culture of talking about not having children/justifying delaying/etc you now have 'not having children' as the 'catching' social outbreak.

Paying people to have kids/social promotion has not changed things anywhere. Or in the case of India being discussed, improving conditions have resulted in less children. There is something else going on than your assertion that 'American's are just too aspirational' is impacting India's fertility rate.