What makes you say that?
You're right to point out that there is a bit of a conflict between Soylent's two goals of being both a viable replacement for every meal, and for any given meal. If you only eat one meal of it, it should probably counteract the bias of your other meals, namely, having too much carbs. But keto-Soylent would not be ideal for everyone using it as a complete food replacement.
http://files.soylent.com/pdf/bar-nutrition-facts-en.pdf http://www.questnutrition.com/protein-bars/choc-chip-cookie-...
Granted, they are trying to do different things; Soylent tastes like crap but has a lot of nutrients(supposedly a "complete" meal), whereas Quest is higher in protein and lower in sugar while tasting terrific.
Personally, I would still choose Quest over Soylent because I suspect that the idea that we need a constant intake of a wide variety of nutrients isn't really true. I'm not denying that we definitely need vitamins and minerals, but experience of going months eating very few vegetables(and almost entirely meat and cheese) has caused no noticeable health effects in me besides weight loss. We're made to think that we've got to eat a ton of vegetables and even take supplements, which I think might be more clever marketing than anything else.
Therefore I don't really see the place that Soylent is trying to occupy and I can't help but dismiss it as another nutritional fad.