You've mentioned some good points in your article.

One reason for having many kids in agriculturally predominant economies like India was and still is was the fact that more kids = more hands to work the fields. Sadly this probably contributed to female infanticide besides your point on legacy.

Have you considered medical advancement as a factor? With lesser infant mortality and the ability to have children at a later age now reduces the need to have too many. Today India's infant mortality rate has reduced to 23 from 189 deaths per 1000 live births since 1950.