[Update a couple minutes later]
Here‘s a piece from Slate that I missed a year ago. No one seems to offer a single reason not to go this route, other than “it requires study.” It’s been obvious to me (as I said) for years.
I’ve got mixed feelings on this. I agree that they shouldn’t be given full human rights, but I do think we should make distinctions as a function of intelligence, and perhaps even recognize some degree of moral agency (e.g., dolphins). It also raises the issue of how we would treat extraterrestrials as a function of those things.