I generally agree with this reasoning. You'll notice each member of society had their ability to contribute go up by 2 units. So the overall gain was 20 units,

I generally agree with this reasoning. You'll notice each member of society had their ability to contribute go up by 2 units. So the overall gain was 20 units, and the overall loss was 10 units, for a net gain of 10 units of "ability to contribute".