It is the ultimate rationale for all genocide. I am not saying that future lives are unimportant. It seems appropriately careful to act as if we hold the earth in trust for our descendants. But it is a huge leap from this proposition to the one that we must injure existing people to protect our descendants. We can concentrate our behavior as trustees on encouraging population control while distributing resources fairly to people in existence.
Compassion never involves a cost-benefit analysis.
I am not saying that compassion should never be tempered by justice, or limited by the possible, nor am I advocating that we should all kill ourselves through compassionate behavior. But the dam does not form part of the river, and the cost-benefit analysis does not form part of compassion. There is something fine in most of us that recognizes that anyone in danger is of paramount importance. This is the ethic of the emergency room, where we behave no differently if the man with a sucking chest wound is a policeman or a murderer. This is also the ethic of the New Testament, which says that it is more important for the shepherd to seek the one errant sheep than to remain with the 99 safe ones.