OK. So suppose that I grant your claim that donations to sub-Saharan Africa will not substantially affect the size of the future economic pie, but that other investments will. I claim that there may still be reason to donate there.

I grant that such a donation will produce fewer dollars of value than investing in capitol infrastructure. On the other hand dollars is not the objective, utils are. We can reasonably assume that marginal utility of an extra dollar for a given person is decreasing as that person's wealth increases. We can reasonably expect that world GDP per capita will be much higher in 100 years, and know that GDP per capita is much higher in the US than in sub-Saharan Africa. Thus, even if an investment in first-world infrastructure produces more total dollars of value, these dollars are going to much wealthier people than dollars donated to people today in sub-Saharan Africa, and thus might well produce fewer total utils.