The human body is incredibly complicated and we really don't know what we don't know. It's really not necessarily true that drinking a nutrient rich slurry like Soylent is a good substitute for a conventional healthy diet. It literally is an unknown. There's good prior reason to believe it is not significantly better. It probably isn't a whole lot worse either, certainly better than many people's diets.

Thus, unknown benefit or harm which probably isn't huge either way. It's like taking an untested drug that probably isn't dangerous but probably doesn't have a huge effect either. The precautionary principle is the usual rule with such things.

To really know, we'd need multi-decade longitudinal studies of large numbers of people. Soylent obviously doesn't have that kind of evidence.