But China has a way bigger market than Russia to successfully go it alone. Why do you think Boeing and Airbus depends on that market so much? If they just sell it to the US and European market alone, that would not be sufficient profit to sustain the program.
Now as to why Russia and China are partnering on this project? I don't know, probably political. Russia needs China economic clout and China's good relations with the EU and US without any sanctions to sale airplanes parts and services from. Otherwise Russia is under major economic sanctions that prevents it to be able to access not only major market, but services and trades as well. The question for Russia is keeping the annexation of Crimea worth it?
it has nothing to do with that, everything is ticket price and safety.
Airliners buy aircraft that their clients fly, airliners charge money, the ticket price is related to the ability to recover the aircraft purchasing price, maintenance costs, operating costs, then you get an airfare.
Why Aeroflot did not use Tu-154s or Tu-204? the answer is simple Airbus aircraft offered cheaper prices, so if you compete with another airliner you can offer better deals then you get a better plane.
People buy tickets upon two factors can i pay it? can i afford it? and the safety factor, will i come back from my trip healthy and without any trouble?
If you use logic flying in a Tu-154 is the same thing than flying on a B-787, both fly, but the difference is the safety factor, ticket price, operating prices, etc etc.
China can force COMAC aircraft upon the domestic Chinese airliners, yes they can, but is it better? i mean economically speaking?
On large aircraft the investment is more expensive, thus it makes sense to COMAC to partner with UAC.
In the domestic sector flying smaller aicrraft they think they can build aircraft to compete directly with Boeing or Airbus, the COMAC C-919 and ARJ-21.
You can have the market, but if you do not have the better plane it does not matter, when American companies need an Airbus they buy it because of all those factors and same is European companies, if they need a Boeing well they buy it.
Chinese companies follow the same principles, and COMAC know the risks, thus partnering with Russia makes sense
The only political aspect is both Russia and China are in disadvantage with respect Boeing and Airbus in the large sector, and Embraer and Bombardier in the smaller aircraft regional type market.
Both think they can take some sectors with their domestic products but not against the B-777 or A-350 type aircraft. So they decided to join forces with each other, in some sectors they are rivals, in other they are allies and partners; everything is money