Why isn't meat a sustainable source of protein?
And yes, I'm pretty much aware of current industrial farming processes and what that does to the environment, but maybe you're missing the forest from the trees, as in maybe it's industrial farming that we should get rid of. Plants and animals are basically engines storing and converting the sun's energy into nutrients and natural ecosystems are very much sustainable. Add fossil fuels in the equation, like what the industrial age did and you get the current clusterfuck. Our grandparents seemed to have no problem in raising their own livestock with sustainable practices. Surely we can do better than them, since we live in the information age and all that.
And for people worried about efficiency, cost and over-population, we are wasting about one third of the food we produce right now. And we spend much less of our income and time on acquiring food. There is room for taking a step back in terms of throughput and maybe focus on quality rather than quantity. Which might be a good idea given that even though the calories in our food is at an all times high, for the first time in history we're seeing overweight children suffering from malnutrition.
The real problem I'm having with your argument, as it's what vegans usually say, is that if you want cheap, you can't beat a combination of corn and soy grown through an industrial process, as these are the most efficient farmed plants we have. Whereas many fruits are very inefficient, consume a lot of water and are not sustainable at scale. Just saying, if you want efficiency, start eating corn.