And even if we did optimize it for healthiness, your argument is essentially saying that we might very well screw up and acquire deficiencies even if we tried to ensure that we got everything that we needed.... and that argument can be applied to normal diets just as well as Soylent.

I disagree. A random basket of foods, while likely deficient in something or other, will also likely change what it's deficient in over time, while Soylent by definition will be consistently deficient. Even if regular food vs Soylent were both equally harmful, the harm from the regular food may be less due to the variability of it. Statistics analogy borrowing from Jaynes: total error in an sampling estimate can be broken down as random error vs systematic error/bias - but random errors gradually cancel out as the sample size increases, while systematic errors remain the same. Soylent is all systematic error.