We are adapted to obtain nutrients from food. Since we currently lack a good understanding of exactly what properties of food are nutritionally relevant, it seems unwise to replace natural food with artificial food.
Yes, processed foods are quite unnatural. But Soylent is even less natural than processed foods, so this is irrelevant in the present context.
From an evolutionary standpoint, legumes, milk, and grains are "artificial" food, at least for humans. Agriculture is a recent thing. Would you also endorse the Paleolithic diet movement?
(I do actually endorse the paleolithic diet as probably optimal at the moment and I agree with your central point - I just want to point out that even unprocessed modern diets are already rather unnatural.)
Rob Rhinehart's food replacement Soylent now has a crowdfunding campaign.
If you're interested in one or more of these benefits, send in some money! There is also a new blog post.