Good discussion. In truth, you can be low impact or high impact regardless of your diet choice.
Same as livestock's food requires water, soil does not just magically produce food every year. Growing plants requires fertilizer input, lime, tillage, fuel, equipment, weed control, etc., etc. to maintain production, all of which require resources and cost $$.
No fact here, just a personal observation ..... The all veggie diet will require a lot more toilet paper too, meaning you are killing more trees :)