1) Graham found philosophy not useful and didn't understand it. This is basically a sample size of 1. Is this a meaningful sample or just a confirmation bias (or suppressed evidence)?
2) Graham believes the early philosophers were encouraged by the progress in maths. But was there really much progress before Euclid's Elements, written around 300 BC, one of the "the most successful and influential textbooks ever written"? Sokrates lived about 150 years earlier (c. 469 BC–399 BC), Plato 50 years earlier (428/427 BC–348/347BC) -- even Aristotle lived earlier (384BC–322BC). One might as well argue that the Greek tradition in philosophy encouraged Euclid to write his elements. The Elements might also have been inspired by Aristotle's logic. But that was obviously useless, according to Graham.
3) In hindsight, it's easy to say what's, for instance, the shortest way out of a labyrinth. But one needs to go many false routes to find out. Are these useless, then? Graham obviously thinks he could spot the shortest way out, by studying "useful" goals. But who really knows? It's easy to say: "I know what's the shortest way" but most people fail when put to a test. Even Wittgenstein was first "wrong" and wrote a rather abstract -- and Graham would probably say: useless -- treatment called "Tractatus Logico-Philosophicus" before he was able to articulate his critic of language. Sometimes you need a ladder before you can throw it away.
4) Graham also forgets that philosophic thinking inspired or formed many other schools of thought: for instance, the study of moral philosophy by Adam Smith marked the beginning of modern economics -- indeed, a useless art --, the study of natural philosophy and that of Malthus' work (an economist and demographer) lead to the discovery of "natural selection" by Darwin -- truly useless, I guess -- , the study of the philosophy of positivism by Comte started modern sociology -- yet another useless art --, theories guiding modern scientific thinking were formulated by philosophers (Popper, etc.) -- obviously useless, too --, even Russel and Whitehead's work was basically inspired by philosophy. And these are just the proverbial tip of the iceberg. Anybody who's a little bit careful can find these examples easily. Graham, however, failed. What does this tell us about his essay?
In summery, what he wants philosophy to do -- namely concentrating on useful stuff -- is already happening, for hundreds of years. It's called science.