If we switched to a 100% plant based diet, there would be a strict decrease in the amount of available food. Animals eat things besides food that could have gone to humans. Land that could be used for growing crops is rarely used for raising livestock.
animals eat lots of our crops eg 80% of the worlds soy.
the vast majority of the soy eaten by animals is the waste product from soybean oil production. that's a conservation of resources, and it's a good thing.