Selfishness and greed are fundamental components of human nature
I don't think it's simply a matter of selfishness and greed, more a case of myopia. Morally 'responsible' people, also, are carried along on the crest of civilization's gigantic tidal wave toward self-destruction. As you say:
How is it possible to convince a man or woman to have the same concern for a generation five times removed as that man or woman has for their own progeny? I suspect it is not possible, but it does seem to me to be necessary to accomplish the task of stewardship.
Perhaps a compelling sense of stewardship must come through religion.
Certainly not through a religion that advocates large families and opposes contraception, for example. There's a very dangerous rationality deficit in that. Which religion did you have in mind?
Rationality appears to be insufficient...
Rationality is our only chance of survival. We need to be made conscious of what 'rational' means in this context. Let's hope against hope that that awareness comes soon enough.