Energy isn't the only consideration, you also have to think about nutrients. Animal proteins have all the amino acids we need, but the same can't be said of any plant. Essential nutrients like Iron, Calcium and Zinc are also difficult to come by for vegetarians. Vitamin B12 can't even be found naturally in non-meat foods, and it's necessary for our bodies to create red blood cells.

Getting the energey and nutrients and having a variety of foods to choose from (so you're not eating the same thing every meal of every day) is not an easy task for vegetarians. Hopefully we'll have supplements that can compensate for that in 15 years, but it's not exactly something you can rely on when planning something of this scale.