Do not underestimate the degree of shamelessness of the hegemon. Since Trump’s last presidency, the US has sanctioned numerous Chinese companies for merely “supporting the modernization of China’s military” or “being linked to the PLA”. Even when you are not using any American suppliers, being cut off from SWIFT and the America’s monetary system is not fun. Norinco is under such entity list and they have to find really innovative ways to circumvent SWIFT for settling international payments in their arms export deals. I’ve sure COMAC, being a company specifically separated from AVIC to handle civilian business, will not be keen to suffer similar fate.
On the point of Western specialized aircraft using civilian airliners as their flying platforms, first and foremost Boeing do not need to worry about sanctions. Secondly, the 737, 767 and DC-10 are mass produced mature platforms that are relatively cheap to acquire as they benefited from the economy of scale. Fuel efficiency is a nice bonus to have, but the main goal for such aircraft is actually range and endurance. For land based AEW&Cs and tankers, we can always use a larger aircraft with bigger fuel tanks to achieve the same endurance. In fact, the Y-20 is a much larger aircraft than the C919 with more range than even the proposed C919ER version. IMO the risk of sanction vastly out-weights the minor saving in fuel cost, which is exactly the reason why every post cold war non-Western AW&Cs and tankers are based on military transports when non-western civilian options like the Sukhoi superjet and MC-21 are available.
