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.
For meat you need you grow the meat, and you need to feed the people growing the meat, and you need to feed the people growing the stuff that you feed the meat. That's a lot of infrastructure. And the only benefit is that protein, vitamins and minerals are mildly easier to get.
A healthy vegetarian diet could be more diverse than a meat-eating diet, especially if you have the limitations.
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.