This is the real answer. Nature only cares that the old already mixed their genes. So, from an evolutionary biology perspective, it's good to clear out the old stock at the end of every generation. Nature already got whatever it was gonna get out of us at that point. To allow that set of genes to stick around actually yields negative returns, ie - less chance of producing improvements, at that point. (Again, just from the perspective of "Mother Nature" so to speak. Obviously the animals themselves would rather not die.)
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This is the real answer. Nature only cares that the old already mixed their genes. So, from an evolutionary biology perspective, it's good to clear out the old stock at the end of every generation. Nature already got whatever it was gonna get out of us at that point. To allow that set of genes to stick around actually yields negative returns, ie - less chance of producing improvements, at that point. (Again, just from the perspective of "Mother Nature" so to speak. Obviously the animals themselves would rather not die.)