"I saw an article that stated the C-17 on average uses about
20 % of their payload capabilities. "
Any source, what criteria was used ?
N500, luckily I found it back, its for USAF 2005/2006 and even less: 17%. And 22% for the C130.
Quiz: How loaded do US Air Force transport aircraft operate? | The Blog by Javier
Doesn't mean you shouldn't have the capability (ask the french..) but knowing the direct operating costs of big aircraft it sure makes a case for having aircraft like the C-27..