Many folks wanted the C-17 but not at the reduced capability the Pentagon required to produce the civilian version. It was never cost that shot the deals down - it was always whether the Air Force would give up the technology the company's wanted to keep in the plane. Guess who won?
Considering the Air Force retired fourteen C-5A's, why couldn't private investors buy or lease them for use as outsized cargo planes? Those include the first eleven C-5's built and three that had troublesome maintenance histories. One of the retired planes has been completely torn down to and is still being studied get data about the condition of the planes in the fleet and their long term viability.
The remaining 112 planes (60 C-5A's, 50 C-5B's, and 2 C-5C's) are being refitted with new glass cockpits in the C-5 Avionics Modernization Program (AMP). After the AMP the C-5B's will go through the Reliability Enhancement and Re-Engining Program (RERP). They will get new GE CF6-80-C2 engines similar to the ones on the 747-400, new pylons, new auxiliary power units and upgrades to other systems in order to improve the reliability of the C-5. Depending on how well the RERP program works for the C-5B and how much airframe life is left in the C-5A's based on the tear down studies, the C-5A's may also be upgraded.
Considering there are thirteen C-5A's that were all rewinged in the 1980's sitting in the desert, I would think it would be possible to get private investors to pay for some of them to be updated with the AMP and RERP for use as commercial large cargo aircraft.
The whole purpose of the BC-17 program was to increase the number of C-17's available during a national emergency without having to paye the costs of operating and maintaining them during peace time. I read somewhere that the Air Force is considering allowing some of their high time C-17's to be leased or sold to private air freight companies and replaced with new build C-17's. That would increase the number of C-17's built while giving the Air Force the youngest ones. I doubt air freight companies would fly C-17's as rough as the USAF does. Few of them would ever need to land on dirt or gravel runways.