And this is why speciesism is a root cause of so much human-on-human oppression. People unquestionably accept the premise that it's ok to exploit/murder/etc non-human sentient beings, so dehumanizing certain groups is incredibly effective at getting people to be ok with abusing them. If we rejected non-human abuse as well, there would be no incentive to dehumanize each other
I mean, you sort of have an argument there. But that's also a really huge leap: "If we can't stop people from refusing to value other people, let's just get them to value every living thing."
If you're at point A on the line, you're going to have to get to point B before you get to points C, D, or E.
it depends how and why they're devaluing the other human. If they're basing it on dehumanization in order to exploit/abuse them, then that is in fact built on the underlying assumption that nonhumans are fair game to exploit/abuse, although they are also factually incorrect about the other humans humanity.
Strategically, it might be easier to get them to recognize that fact, or it might be easier to get them to accept compassion for all sentient beings and then point out the logical conclusions of that, it really depends on the person and situation, but I'm not talking strategy I'm just pointing out an often-unspoken root problem
Malice and disrespect would still be incentives, but how would it be profitable to dehumanize other humans if it didn't grant you any ability to exploit them more than you could exploit a "humanized human"? What reason would religion have (not that they'd need one to just make it up 🙄) to dehumanize other humans if it didn't imply your religious group is more valuable than the dehumanized humans (besides malice and disrespect)?