I'd tended to believe that the people designing the NAC-60 also looked at the B-70 and saw a number of things they didn't want to do again. Maybe for them, using compression lift was a mistake. One thing not to be repeated was the complexity of the Valkyrie's landing gear. I'd say that to go from the Valkyrie to the NAC-60, they were looking to simplify things. The military tends to tolerate maintenance costs the airlines wouldn't put up with.