There are plenty of medical sole source of nutrition products available, catering to a range of different dietary needs.

That they don't advertise to the general public; or don't say "you can live off this and nothing else" says something about Soylent's unethical marketing.

The Ensure range is easiest to get hold of for the general public. (Ensure and Ensure Plus are sometimes used as sole source nutrition in forced NG tube feeding for hunger-striking prisoners or eating disordered patients.)