My wording in the grandparent was misleading. This:

The 4 month old will produce 80 years of utility if saved, for 18 years of investment: if this is a bad bargain, reproduction always is.

Was meant to refer only to having a baby in current conditions regardless of whether or not it would be sacrificed to save an adult. I meant that if 80 years of production minus 18 years of investment was a net negative or a net positive, that wouldn't change whether you saved the adult or the baby.