I guess where I struggle with your point is 1) why having no idea what aliens are like means we can't give grace in terms of our interpretation 2) What arguments you think are possible to exclude humans if we expand as I talk about.
My reason for 1 is just for the sake of being thorough and making sure we understand what we are extending this term to mean because when we do meet an alien species or if we recognize a species on earth as having moral agency and being sapient, I think it isn't as simple as saying it is automatic that we treat them as humans (again see my AI example). We just don't know what the context will be and what the nature of these beings will be, because we might not yet have imagined that.
For 2 my concern I think is well founded. If the default definition of personhood doesn't automatically include human beings (and not saying you are making this case), then you can easily exclude people by saying they lack moral agency or free will or they are not fully sapient (again this does depend on how Personhood is defined). But if it is just two points like that, and doesn't also say something like "and anyone who is a human being" then there most definitely have been arguments in the past that could put people like the disabled, the mentally handicapped, people with serious mental disorders that impact their ability to make informed choices, etc. And in the past there have been whole groups of humans not fully recognized as people (like women, and slaves). So I just think if personhood is going to be the thing that marks what protections people get, then you have to include all human beings in that.
Also I will say I am a lot less concerned about this point than point 2. This is something where I get the moral concern and think it is valid. But I do worry about a definition of person that isn't linked to humanity very closely
I mean, sure, you can argue free-will doesn't exist, but then that applies to all humans and likely all people. There isn't a specific instance that denies free-will to one person, but keeps it for another.
You and I might agree this is the case. But I can bet someone will make an argument that a person with significant enough brain damage to their frontal lobe who lacks impulse control doesn't have free will in that sense. Or someone int he grip of a drug addition doesn't have free will. Or someone suffering from the effects of Alzheimer's disease. You an easily see people making arguments eugenics and social darwinism using these kinds of reasons. Which is why I think our personhood should be viewed as inherent and obvious.
And if it is, it would be like trying to argue that since the definition of human include bipedalism, that someone who was born without legs isn't human. That's obviously false, because definitions of things such as this are obviously going to have exceptions for exceptional situations, but that doesn't change the baseline to allow discrimination to be okay.
Except being a human doesn't require having both your legs. Being a person, at least how this term gets used in conversations around ethics, very much depends on you meeting the criteria individually. That is one of the reasons the debate around personhood is so contentious
