China can force to the Chinese airliners to buy Chinese made aircraft, but outside China things are different, the US and European Markets are big too, but you forget that even the Latin American or African markets are important as a whole, Russia also forces Russian airliners to buy Russian made aircraft.
Larger aircraft are built in smaller numbers
B-747 less than 1600 in 47 years
B-777 less than 1500 in 23 years
B-767 less than 1100 in 35 years
these aircraft are built for the world stage, for Europe, Asia, Africa, South America etc etc, Russia has built less than 150 Il-86 and Il-96 since 1980.
Do you get it or not
A-340 has less 400 aircraft made
These aircraft are built for airliners around the world.
Aircraft need markets otherwise the profits do not come and the risk of not even recovering the investment exist, basically China will not fix that by forcing its airliners to buy Chinese made aircraft
Do you think Il-96 has recovered the investment and made profits with less than 30 made?
Why do you think MD got bankrupt? do you want the answer? well MD-10 was not as successful as B-747, Convair also made the 880 and 990 were not success stories, same L-1011, well not even B-146, if you want to know why Airbus, Boeing, Bombardier and Embraer are the most successful makers, see how many aircraft have made and sold.
Russia despite is a minor player in sales, has aircraft like Tu-154, Tu-134 and Yak-40 which have been produced in larger numbers than any Chinese jet, and more important Russia has the jet engine experience to compete with western manufacturers.
the Tu-154 went beyond the 1000 aircraft made mark, China is struggling to sell the ARJ-21 and it is forcing Chinese made aircraft to the Chinese airliners.
Do you think Germany did not make airliners after WWII, well they did but this never went beyond a few aircraft made, the VFW-614 for example never reached 111 aircraft made only 110 were made
Why they joined Airbus?
simple larger markets give more chances to any aircraft a Chinese Russian made aircraft might be bought by Cuba, Belarus, Iran, Kazakhstan and some former soviet states, if lucky this will lure African or other Asian nations, so Russia is a partner too for world stage markets
How would partnering with Russia open up Cuba, Iran, et al? China has good enough relationship with almost all of them to garner orders without a Russian partnership. I didn't forget about Latin America or Africa, I just don't see how Russia can help in any appreciable manner for opening up those markets considering Chinese presence there already far outweigh the Russian one and the difference will only become starker by the time the 929 is ready. BTW, ownership of a plane isn't shared by the countries it flies to, China doesn't need to sell planes to Africa to fly their planes there, it just needs the planes to be certified to fly there, which again, I don't see how Russia can help.
Yes Russia still has some tech that China could use, but I repeat, there's no way in hell Russia would give them up.