This is an excellent summary. It's not about a "fad" or being "organic" or something like it.
Nutrition is a very complicated subject and while science has shone a lot of light on the subject we still don't know a lot. So sticking to the "tried and true", that is, what people have actually been eating is safer.
This is a valid point to an extent, but we don't know that people do fine on a particular subset of traditionally healthy ingredients. We may discover that Soylent lacks some subtle nutrient, but we could discover the same thing with MealSquares.
But we don't really know how far along the scale between dead and in maximum possible health traditionally healthy food gets us. Short of some genuine scientific results, I see no reason to suspect that Soylent is less healthy than traditionally healthy food. It could just as easily be more healthy.