I'm writing the arguments and questions below in good faith. I'm legitimately conflicted over this, and I admit that I haven't engaged in any research on this subject.
From what I can tell, the moral argument that you're presenting seems to boils down to the fact that we arbitrarily value animal life more than other kinds of life. I'm not saying that this is somehow "wrong" -- I'm bothered by the fact that the it's often presented in a way that makes it seem like it's some kind of universal truth, based on some criteria of the subject itself (e.g. presence of certain biological structures or phenomena), rather than specifics about the human experience.
> Plants can't experience pain, at least insofar as we conceive of it, because they don't have nociceptors that signal pain. Therefore, if they do have any sort of phenomenological experience, whether it would be recognizable to us or not, there is no sense in describing anything they experience as "pain".
So, they lack the biological machinery to feel (let's call it) animal-like pain. Is the argument that it's our moral obligation to reduce the phenomenological experience of animal-like pain in this universe? And by extension, that other kinds of "pain" (for lack of a better word) are less important? If so, isn't it more to-the-point to simply say "we shouldn't hurt things that are like us"? (This simple perspective is obviously problematic in its own ways -- I'm not necessarily advocating for it -- I'm just trying to clarify the essence of what I'll call the "nociceptor argument").
> A final point on the deficit of nociception, is that the experience of pain is so central to our lived experience that it's difficult to conceive of what that would be like. It's like the alpha channel in rgba... Without that signal for opacity, there's no color at all.
The argument here seems to be something like: "it's incomprehensible to humans, so therefore it doesn't exist." Again, it seems more straightforward to simply say that the fundamental criteria is whether or not the thing experiencing "pain" is sufficiently "like us".
I think basically what I'm trying to say is this: the "nociceptor argument" feels like a way of skirting around the fact that morality regarding animal rights boils down to human-specific feelings we have about animals -- and that those feelings are rooted in the fact that they're like us, and some quirk of our psychology causes (most of) us to feel pain when we see or imagine something like us in pain.
I personally don't believe that reality has hard boundaries of the kind that the "nociceptor argument" presupposes. Humans conceived those boundaries because they're useful to us as a part of how we model reality. From your response to another commenter ("...the last is not really the type of argument that I find fascinating beyond its relevance to social ritual or linguistic games"), I imagine you might find these arguments to be too metaphysical for your taste, but in my opinion, it's important for people to realize that these moral frameworks are rooted in human culture and psychology, not some biological reality independent of our species.
I'd be very happy to hear your thoughts or rebuttals, if you care to share them.