Completely correct, however gains through increases in CO2 aren't going to be much use if over a, say, 50 year period your good crop growing climate changes significantly. For example, you might have an increase in CO2 levels, but a decrease in average rainfall. There may be a tech fix, but chances are viability would be reduced. In any case the broader environment isn't a closed system, and the point being made is CO2 isn't the only variable changing, for good or worse.
