How the supply and demand thing works is that you don't buy animal products, so there's more supply than demand, so the makers can't sell all their stuff or can't sell it for as much money. After awhile some of them get discouraged and do something else for a living, reducing the supply so it meets the demand again.
The difference between the original supply and the new, lower supply represents the critters that didn't get offed to feed you.
I can't make a huge difference perhaps, but I can make one person's worth -- and that's what I am responsible for.
Does one person having beans instead of chicken make any difference? If you could ask the chicken I'd guess so. It's like the saying that all wars are the same size, because death comes one to the customer.