I don't think anyone is arguing that Soylent could replace breastfeeding (though there are plenty of people who use something similar: formula).
But for everybody else, Soylent sounds a lot easier to customize to personal needs. You can avoid allergens without losing the required nutrients, you can tune the amount of sodium and potassium to whatever you can deal with, etc. With normal food, you don't get that kind of fine-grained control.
You simply can't rely on a one-size-fits-all diet without some risks. One person might be lactose intolerant, another might have nutritional deficiency, and a third might be at risk of overdose due to genetic or medical factors preventing them from offloading excess amounts of Sodium, Potassium, etc.