OK, but then you still have the problem of how to balance the needs of the people who already exist. For example, I really like animals. I place a very high value on the possibility that I might some day see a Sumatran rhino in the wild. If Sumatran rhinos go extinct then I, a human who already exists, will be very sad and have a lower quality of life. How do you balance that against the needs and desires of others who don't value Sumatran rhinos as much as I do?