I've never been fond of this line of argument, because (A) the concurring opinion of a future person is not a definitive source of authority, and (B) there is no way to verify or falsify such a prediction about opinions in the future.
Not saying I disagree with the overall point, and I personally would not feel comfortable murdering a fellow primate. Just saying I see this "future generations..." line of persuasive reasoning often and it usually jostles me in an unwelcome way.
