Non-vegan playing devil's advocate, here; I've always hated that argument: Yes the chicken is already dead, but by purchasing the product, you're paving the way for more chickens to be killed to replace the one(s) you ate. Supply and demand. We need to come up with a better counter-argument.
Personally I'm not ready to stop eating meat, and probably never will be. I'm counting on lab-grown meat to come down in price enough to be produced en mssse within the next 10-15 years, and then I can enjoy all I want, 100% guilt-free. I will be among the first to buy it when it hits the supermarket.