Based on this faulty assumption, Landsburg then goes on to make the assertion that since our grandchildren are going to be so rich, they won't mind being reduced to seeing things like trees only in photographs, and they might prefer inheriting the proceeds of economic development to inheriting the redwoods, anyway.
All trees will be gone once our grandchildren are grown. 
mnli hypothesis: All trees will be gone once our grandchildren are grown.  premise: Based on this faulty assumption, Landsburg then goes on to make the assertion that since our grandchildren are going to be so rich, they won't mind being reduced to seeing things like trees only in photographs, and they might prefer inheriting the proceeds of economic development to inheriting the redwoods, anyway.