Fact number one Y-20 flies with russian engines and China did a large order of those same engines.
Fact two C919 has at this moment a Soviet style market, mostly Chinese airliners are buying the product.
Reality number one, if Y-20 is only bought by China with 0 export market, the aircraft is unprofitable, why? let me remind you military aircraft do not represent a gain if they are financed by a government that also buys it.
Why the Soviet Union bankrupted ? simple weapons are only expenses, why Russia only exported during the 1990s and most of the 2000-2010 decade? simple the Russian government was unwilling to have expenses so Russian manufacturers went for profits.
Is Chinaa wealthy nation? reality 2, No China has still hundreds of millions of people with a low income, and with still standards of living lower than all western developed nation and still lower than even most emerging markets.
Does China needs to expend money in social programs?
Yes of course, so buying Y-20 for PLAAF does not mean any improvement of the quality of living for the Chinese citizens as the Soviet Citizens did not get inprovement for having a powerful military machine.
So does China need to export? yes they do
Why joint ventures are important simple it opens new markets to your products?
Why western and Japanese manufactures have joint ventures with Chinese firms in China? to open the Chinese market.
why western engine manufacturers cooperate within them selves?
same reason lower risks, costs in R&D and open new markets
The Engine Alliance, a 50/50 joint venture between GE Aviation and P&W, was formed to produce an engine for the Airbus A380. The Engine Alliance competes directly with Rolls-Royce for A380 engine business and holds a roughly equal market share with Rolls-Royce
International Aero Engines AG, a consortium comprised of P&W, Rolls-Royce, German engine manufacturer MTU Aero Engines GmbH and the Japanese Aero Engines Corporation, produces the V2500 engine for use in the Airbus A319/A320/A321 aircraft.
Fact is China needs to develop an independent Air Transport capability for 2 reasons.
(1) Russia is has proven to be an unreliable supplier as proven by the il-76 debacle.
(2) US cannot be counted to provide humanitarian assistance to China when it matters most as proven during the Sichuan Earthquake by her refusal to provide parts to Chinese Black Hawk Helicopters.