IMO, C929 is a R&D/political project, not so much a economical one. The fact that it envolves 2 different countries with 2 different R&D centers creates a lot of development risks (political, financial, one side delaying the other, etc). Also, i suspect that russia will hesitate in sharing much of its know-how with china, and that this could decisively hurt the project.
it is economical rather than political.
Russia and China went separate ways to built C919 and MS-21, why? the economic risk is lower, the markets for both aircraft are large, for aircraft in the range of B777 or even B-787 is much smaller.
More seats per airplane mean less aircraft sold.
China if lucky could built 1000-5000 C919 aircraft in China and overseas, but C929 or C939 if they are larger, fewer aircraft are going to be built.
Airliners buy large aircraft for long distances and smaller for shorter routes.
B-787 is a big success, it has sold 500 aircraft already and a larger aircraft built by Russia for example Il-96 can not get even close to B-777.
China has no chances, for starters, China has not even built ARJ-21 well, the jet is mostly sold in China, Both China and Russia can not enter the large aircraft markets without big risks.
Russia has not penetrated well the Market, and by that i mean the western or even African one.
Tu-204/214, a B-757 type aircraft has not even been successful less than 150 are in service, the Tu-334 a kind of ARJ-21 was not even built beyond a single prototype .
Is not that Russia lacks the technology, Russia has the technology, but aircraft are bought by very small margins, if an aircraft has 4% or 5% better range, 6% better economy etc etc.
Plus you have the fact commercial aircraft are bought because political reasons, Japans buys B-777 because they build the wings.
Russia and China know they can not compete without the risk of failing, thus Russia gets access to the Chinese Market, China gets a potential share partner for the world stage sales.
The reason Airbus was created was because all the European nations could make aircraft, but all these projects had the risk of bankrupting their makers.
Same is Russia and China.
Small nations help the big players in ways people do not expect , for example Frisa from Mexico and especialistas en turbopartes from Mexico too both supply Airbus and GE with small but fundamental parts in aircraft engines or landing gears.
China and Russia will do the same some Chinese companies will supply parts for the joint venture of COMAC and UAC and some Russian companies will supply other parts, is not that they share tech, they simply agree to supply parts to the aircraft in the same way Spain or Germany build different parts of airbus aircraft but then they need to coordinate to design them so they fit well.