This was my response when David Brin made the religious argument: monastic or hermit diets are not famous for their variety or density, and one of the most common religious strictures is no meat - which immediately makes protein difficult to obtain.

Getting adequate complete protein on a vegetarian diet is not at all difficult, and in fact, usually does not even require attentiveness to one's food intake. Getting adequate protein on a strictly vegan diet generally does require attention to one's food intake, but is still entirely practical.