I thought part of the problem was that McDonalds /did/ suggest you /could/ do that, without any ill-effects - that their meals were healthy and you could eat breakfast, lunch and dinner there without a problem.
On the other hand, soylent is making the claim that it's healthy to use soylent as a complete replacement for food. Thus, the standards are much higher in terms of truth in marketing.
http://www.mcdonalds.com/us/en/food/food_quality/nutrition_c...
There's no mention of food from other places, and the final paragraph ("Variety keeps things interesting") certainly implies you could eat there daily without problem, to the exclusion of other food sources.