As a vegan myself I think appealing to empathy is probably counterproductive because it implies that meat eaters don't have empathy. It's basically the vegan-mirror-equivalent of slothtrop's "Nothing's ever enough for vegans." comment. I feel like "meat eaters don't have empathy" is the common judgment that vegans cast on meat eaters, and "vegans are unrealistic, holier-than-thou perfectionists" is the common judgment that meat eaters cast on vegans.