I have very strong priors against this idea. The priors are based on the following:

• Our knowledge of human biochemistry is very incomplete. We have only a vague idea of how a huge variety of substances that we normally eat on a day-to-day basis affects us. Studies showing effects of various compounds on human health pop up (and are shot down) all the time. I am not willing to accept that we know enough to construct a complete diet from molecular building blocks.
• The goal of choosing food (besides sensory considerations which are clearly not important here) is to be healthy, not just stay alive. What would be the effect of living on Soylent on your heart disease risk? Cancer? Autoimmune diseases? Thyroid? what, you have no idea..? :-/
• People are different. It's very clear that there is no single diet suitable for everyone. Is Soylent suitable for rapidly growing teenagers? For old people? For bodybuilders? Pregnant women? Someone recovering from injury? Someone with gastrointestinal problems? There is no average person
• There is a much evidence that societies switching to "contemporary" refined food, notably white flour, sugar, vegetable oils, rapidly acquire all the so-called diseases of civilization. The evidence is epidemiological/correlational, but there is a lot of it.
• We have a large and very important to our health collection of bacteria and other organisms living in our gut. It's a bad idea to shift it towards pathology and I'm pretty sure subsisting on Soylent is going to affect it in major ways.

Now, I'm perfectly fine with experimenting on oneself and willing collaborators. But mass-marketing a "food replacement" to the general population doesn't look like a good idea to me.