i think it's important at this point then to note that we don't just eat food for energy, we also eat it for nutrients. if all we needed was energy, we could survive by just eating sugars all the time. like mczealot said, vegan diets have to be supplemented with nutrients that cannot be obtained in sufficient amounts from plant matter, and the process of synthesizing or cultivating those supplements would probably be more economically intensive than animal-based solutions on a society-sized scale. that being said, i'm talking stuff out of my ass here since i definitely don't have the figures to back that up available, but the point stands that it's not as simple as that contention makes it sound
of course, but like i said: i don't think the meat industry should be axed entirely. not at all. i think factory farming should come to an end (replaced with free-range ranching) and meat should be seen as less of a staple. your body needs about 4-5oz of lean meat a day, but people eat it breakfast, lunch, and dinner.
it's a super unnecessary amount of animals which need to be slaughtered to keep that going, and we've still got millions of people dying of starvation every year. if a good amount of that land was reallocated for staple crops, it seems to me there'd be less starvation and more healthy diets.
'course maybe that's not as profitable for the big boys