It seems to me that the other Pinkie’s would count as people because they displayed a range of emotions, had the ability to think about themselves and comprehend the world, and could carry on normal conversation with the other ponies.
I think it’s important to treat beings with these markers as people, because it probably won’t be too long until we’ve created non-human and/or non-biological beings who fit these criteria, and they should have the same basic rights as existing people. I find that a scary number of people currently living don’t know what makes a person a “Person” with selfhood and rights. They seem to just default to “Did it come from a human uterus?” to answer to that question. It’s not a bad heuristic, but it doesn’t give enough weight to near-sapient non-human animals (who are generally viewed as expendable), and it gives too much weight to non-sapient humans (such as those who are brain dead or severely mentally handicapped).
I think that heuristic is already breaking down a bit. When IVF first became available there was a moral outrage about the soulless abominations that would thusly be birthed; and what legal and moral rights should apply to these inhuman “test-tube babies”. Now it’s seen as a routine procedure, and the moral outrage has instead shifted to human cloning, which is just as idiotic but most people don’t seem to notice. Because they are confused about what a “Person” is, and for some reason don’t want to investigate the issue. So an episode that reinforces this sort of ‘avoiding the subject and just defaulting to the old paradigms’ kinda annoys me.
On the other hand, the new Pinkies certainly didn’t seem as complex as the original Pinkie. They had a maniacal obsession with “fun” and would fixate on that concept to the exclusion of all other concerns. They were certainly less valuable than the fully functional Pinkie. If the population of Pinkies had to be reduced, they were the correct first choice.
Furthermore, none of them protested their treatment. It’s an unfortunate fact that we only have those rights we take, and as soon as these Pinkies realized they were targeted for extermination they should have rioted. Or protested in some way. That they had no concern for their own continuation makes it much easier to justify their elimination. Or perhaps, as Khitchary suggests, they weren’t worried because they knew they were just returning to a different world rather than actually dying. (I’ll avoid the afterlife comparisons)
Anyway, I’m now all done on this subject. To the future!