Of course, you have a valid point. But what you're ultimately arguing against is any form of moral reasoning.

While I find the theoretical discussion of morality interesting, I'm far more driven by the practical discussions. People generally accept a moral axiom, explicitly or implicitly, something along the lines of "Harming other individuals unnecessarily is wrong". If that is the case, I'm going to argue for the consistent application of that principle, which is also where sentience enters the conversation. If they don't hold a similar moral axiom (which I've found is fairly rare), then I don't really have any ground to stand on in this debate. I'm certainly not equipped, if it's even possible, to make a moral argument from first principles.