Here’s a compromise.

Neither you nor I get to decide what the purpose of another sentient being is.

Absolutely. But while I cannot declare its ultimate "final cause", perhaps I have some right to declare a penultimate one? I have my reasons for believing this is the case. What are your reasons for believing it is not (or do you believe that we do have some right to declare penultimate final causes for living creatures, and if so, what are the limits?)