In your opinion, is killing plants deliberately morally bad (especially given some studies I've seen which suggest they can respond to stimulus in a way that could be considered "pain")? Is there any room for utilitarian views on well-being or Nash equilibrium's or some rational measure of capacity for pain / pleasure? Would you say killing an ant is not in the same category as killing a dog as our laws suggest?

you can't make your argument ethical

That's yet to be determined in my opinion just as we are part of a larger ecosystem, and a lion killing its prey is an amoral act. The thing that separates the lion from the human relates to our discussion and how we define morality. It sounds to me you've made up your mind before the conversation has started. I'm saying things are little more complicated than that.

If you want to talk Arthur Schopenhauer and antinatalism, we could do that too, but I prefer a moral landscape approach (Sam Harris) which sees peaks and valleys in morality and us working together to figure what we think about ethics in a changing landscape. If synthetic meat becomes readily available to meet omnivore needs, then I would more actively promote NOT raising animals for the sole purpose of killing them for food (even if they did live a wonderful life according to the capacity of their species to have enjoyment and fulfillment). Until then, I do see how they, as us and the plants and everything else, are part of a system which can be sustainable and ethical.