Hmm. I don't think many people argue that "death penalty is bad because it's so expensive", and I don't think bringing up climate change & animal rights at the same time in the case for veganism interconnects them like you claim.
The consequence is that the proposed framing and solution are orthogonal to what one really wanted to argue -- that death penalty is evil. Instead you find yourself arguing whether the death penalty can be cheaper.
I accuse the argument of "veganism helps ecology" the same way, which is a point you summoned first. People who advocate veganism probably do so for moral reasons, because if ecological sustainability were their motivating value, then veganism would probably be far down the list of problems and solutions. Plus, if they wanted to be practical, they would argue for vegetarianism first, and not the hardline of veganism. It becomes suspect to say that ecology is their motivating value, and that veganism is the solution.
The likelihood is that the veganism proponent is not emotionally or intellectually interested in a discussion about how we might make animal killing processes more ecologically sound, and would find that discussion a little depraved. Now they're caught in a discussion that they're not truly interested in.
But I have been making the case that this is easy to think about with our everyday ethics reasoning without having a rigorous philosophical framework and I don't accept the notion that in the veganism case people are generally arguing backward from a feeling (though I'm sure that also happens). So calling out the death penalty metaphor was connected to my argument that people's ethics reasoning acumen is not as lousy as the metaphor implies.
> I accuse the argument of "veganism helps ecology" the same way, which is a point you summoned first. People who advocate veganism probably do so for moral reasons, because if ecological sustainability were their motivating value, then veganism would probably be far down the list of problems and solutions [...] if they wanted to be practical, they would argue for vegetarianism first
In my experience many people do argue for and practice vegetarianism first, and the studies & analyses I've seen indicate that vegetarianism/veganism are at the top of the list of effective and practical carbon footprint reduction solutions. So I think this thinking is based on false premises.
Re arguing backwards, note that here we were discussing how the average person might consider veganism, and not how the the average vegan got there. So the discussion is vulnerable to something like survivrship bias.