There is no false (or any) dichotomy - extent of nurture clearly implies a continuum on which the couples I used as examples are singular points with many intermediaries on the nurture and survival scales. In the short term your example suggests that the couple with five surviving kids is evolutionarily superior, but that ignores long term survival. The kids from the family where only two survived, have probably shown superior genetic survival qualities. In the extreme where life forms have a 100% survival rate, and all become parents in turn, they would no longer be subject to the forces of natural selection - there would be no picking. Such a hypothetical extreme would not matter in a constant environment, but in a changing one the gradual adaptation required over thousands or millions of years would be lacking. This would then leave such a life form increasingly vulnerable. As one moves away from the 100% survival extreme, adaptability increases. One would of course reach a point where the survival rate falls below sustainable levels.
How important is adaptability to a species that adapts its environment to its needs rather than vice versa?