Precisely so. The large long range aircraft for international flights do not compete with HSR. So there will be high demand.
As more of the Chinese population reaches higher income levels it is quite likely that more Chinese people will travel for tourism. You also have BRICS. The main BRICS nations are fairly geographically distant from each other. Even Russia and China, which are supposedly neighbors, have their main population centers very distant from each other. Just connecting Russia, China, Brazil, and the Emirates via an air bridge will require a fair amount of large long distance aircraft. You might also add Ethiopia, Egypt, South Africa, and India to that mix.
These aircraft could also be used to transport cargo. Which would further increase their desirability.
Air cargo would depend upon type. Lighter low quality production is shifting out of China in its own off shoring into countries like Bangladesh. Higher quality production goods are more cargo container ship or rail transport based. No matter how many C939F China could make. It would never be affordable to ship Geely, BYD or Trumpchi cars and truck on volume upon them.
Air freight and airline passenger industry live a bubble west and east based on any number of political and financial considerations. Black swan events like the Asian financial crisis, 9/11, Covid can drive demand into the floor.
Although tourism is likely to increase assuming a population income increase. The question is whether this would justify an expansion of Chinese Airlines routes into said even then if said increase justifies a new airliner of the jumbo class and not simply purchase of C929 or foreign offerings.
The decision on such isn’t just based on a potential demand but political considerations as well. You can come up with a list based on BRICS but remember just being a BRICS member doesn’t make it necessarily a great tourist destination or even friendly place to visit. Russia is in a war, Egypt is fickle. Brazil and India play both sides. India and China are in a Cold War of their own. Ethiopia and South Africa are politically unstable. Well yes they could be destinations that’s making a big assumption.
Overall I am not saying that there couldn’t be an interest in a C939. I am saying that the safest bet is not to go forward with it. There is a long history of proposed Jumbos that never materialized as resources or demand were not there. MD12, KR 860, Boeing NLA, Tu404, Lockheed LST Boeing Sonic cruiser just to name a few. Airbus almost went bust because of the A380. Boeing is where it is today due in part to issues that came about as a result of the 787 being built with an immature design process.
Comac is just moving its second airliner into production jumping to the success of airliner #4 is premature.