I believe that another element that stymied the X-59 in the 1980s was the lack of a Sugar Daddy. The X-59 was a DARPA sponsored program that had no home in DOD. By contrast the XV-15 had DOD support resulting in continued development, ultimately growing into the V-22. Examples of other programs that similarly went by the wayside are Northrop’s F-20, Sikorsky’s S-67 and until recently the Lockheed C-130J. Privately funded programs with little or no government backing have limited chance for success.