Which, I guess, is ample justification for humans to treat these animals cruelly? Not really following the logic of your argument here. Though I definitely agree with your point about cruelties inherent in the meat and dairy industries.
Just to toss one possibility out, maybe something along the lines of "Hey, that did bother me. Check out this other thing that bothers me about X farming."
I love people who make silly counter arguments. Watched a hunter challenge someone protesting their hunt: "Do you wear leather?"
"I'm a vegetarian" was the entirely predictable response.
(Actually, I now remember it was the other way around. "Do you wear leather" was the next step after asking the protester whether he ate meat and getting the vegetarian answer. As if a vegetarian would go out of his way to avoid meat, protest hunts, and still not be aware where leather came from.)
Restricting one's consumption to vegetables can promote animal welfare despite engaging in other practices that harm animals.
Demoralizing people who strive to promote animal welfare by pointing out their contradictory behavior is less desirable, to my mind, than encouraging people to do as much as they can.
Why not reconsider your own views and encourage such people to promote animal welfare in all the ways they could rather than demoralizing them to give up altogether?
EDIT: spelling, rewrite final sentence, formatting.
I'm asking you to reconsider your position. I understand you're unlikely to do so.
One thing you should consider is the very strong possibility that "the least important human concern" is not distinct from what you consider to be "animal welfare".
Living beings on this planet are probably deeply intertwined and interdependent in ways we poorly understand. For one example, take the growing research around gut flora and microbiomes.
As individual beings, we are crucially dependent on bacteria that, in fact, extend the boundaries of what we consider to be our bodies far beyond what we have understood to be the case for millennia.
Separating the human condition from "animal welfare" is a grave mistake. For your own good, I think you should reconsider your position regarding the care of non-human organisms.
You'd be surprised.