I sympathize with this argument, but the obvious counter-argument is that lots of people have eaten normal diets and have been observed not to, for example, die of scurvy. (On the other hand, they have been observed to, for example, get heart disease.)

That's true. But then again, once you consider that "normal diets" is really composed of countless of different combinations of foods ranging from "fast food only" to "making a constant effort to be trying out new foods all the time", you could also use this as an argument for Soylent being probably safe. As in, "out of all the countless possible combinations of nutritional intakes that people live on, most don't lead to anybody dying of scurvy, so if we specifically construct one new diet for the express purpose of providing everything that one needs, it doesn't seem like it should kill you if all those diets that weren't constructed with that in mind don't kill you".