Jeff, thanks for the wide-ranging thoughts. There is a lot to agree with here, including the points on the decline of research on topics such as taxes and thinking about the welfare implications of public versus private provision.
One thing I am less clear on is how to measure opportunity costs well (or maybe better). If we put a dollar into sanitation intervention X at the expense of education intervention Y, shouldn't we know what the effects (private returns + externalities) are of x and y are? And i think impact evaluation is a really good way to measure them, which suggests to me we don't have enough impact evaluations so that we can get a fuller sense of the tradeoffs we are making (instead of doing the one or two things we know have a positive return).