I'm assuming it for the sake of the piece. I do think that the price of a child's life is rising faster than my investments appreciate, and probably thought they were doubling every 4 to 5 years when I wrote this. (I wrote $2000 back when I posted this to Facebook, I wonder what Givewell's estimates are.)
(To clarify further, this was a post to my Facebook creative writing group in 2015 as was the "Responsibility" poetry I posted.)
This piece isn't intended as an argument against delayed giving (though I think most such arguments would need to deny the premise of the piece). It's a story about not giving. It's about an older man, living in a time where saving a life in Kenya is like saving a life in Canada (that is, out of reach for most people), looking backward. Every year during that short window, he could have been a hero, saving one or more lives. He missed that chance and it doesn't exist anymore.
If you have a way of doubling your money every few years, go for it. But that's rather unlikely.