We hear this every few months, but the real issue is not "what" we eat, but "how" it is produced. Trading one mass-produced food item for another mass-produced food item doesn't get us closer to
sustainability. There are more vegans/vegetarians in the US than ever before, and yet, the number of hungry people grows daily. Obviously, it is not as simple as not eating meat.
Also, we currently produce
enough food to feed the world and then some. The biggest obstacle to providing food for everyone isn't production, it is delivery (which includes policitics, economics, etc).
Animals can play an important role in converting non food items (grass, wild foods,
compost, etc) into nutrient dense food (meat) for humans. In a lot of ways, animals can increase the overall efficiency of a food production system, especially when it comes to nutrient cycles.