Note: I am not vegan, nor I necessarily endorse the thinking behind this answer, but I'm trying to do a service to the discussion by reporting what I heard from a conversation with a vegan friend.

I think the argument here is that most of most animals' worth come from eating them or their products. If everybody were to stop eating animals then it would not be economically feasible any more to use the "waste" parts of the animal for all the other products. We would find other more economically feasible ways to produce the same products (via plant matter or synthetics). Thus, by refusing to eat meat and animal products, all the other uses of animal suffering would cease to exist.