So the argument that research for the sake of research is worthwhile is perfectly sound, the condescending "let me explain to you how a budget works" and "it's not my decision to spend the money" parts certainly rubs me the wrong way.
How is a nun in middle of Africa meant to have even a basic understanding of how a specific government's budgeting and economic system works?
I am willing to bet the vast majority of Americans themselves (to this day) have no idea of even the basics so simply explained by Dr Stuhlinger, yet it was a critical part of the bigger picture.
Still, a good story though. Had he ended there, the letter would lack any real substance to argue his point.
Not just that, but to drive home that although the benefits of research may not be known at the time they are nevertheless existent.
She can't have read a few books on economics? His explanation was pretty basic and nothing that couldn't be found in econ 101 textbooks. He essentially just assume she was uneducated.
If this were in person, he might have been able to ask, "What do you know about the US's method of setting a budget and taxation?" and tailored his response after that. Since it was a letter, greater caution was needed.
Are you offended by the first chapter or two of nearly every programming language book that explains what variables, conditionals, arrays and the like are? If you know it, you can skip it. If you don't, you can read it to have a better foundation for the rest of the text.
It would seem quite reasonable for a nun who received her regular education that early to possibly have grown up going to a religious school somewhere where the idea of teaching topics such as economics to a woman would not have been particularly favoured, and/or where the use of a "standard library" to read texts outside of the curriculum would have been discouraged.
In other words, depending on the wording of the original letter, it might very well have been entirely reasonable to assume that she would not have had any knowledge of economics (for that matter, you could assume a fairly substantial percentage of adults today would benefit from the explanation he gave).