No China can go it alone if it wanted to, it's Europe, Russia, and the US doesn't want to see a real meaningful competitor by restricting the market with FAA rulings. Russian definitely can't do it alone that's why they needed Chinese money. Similar to Russia's T-50 PAK-FA that needed a partner in order to get it the program going and it's NOT doing any better than China's J-20.
China can go alone that is true, Russia can go alone, Germany can go alone, France too, the problem is COMAC does not think like you, in reality COMAC works with UAC, you might not like it, but they go with UAC for a simple reason.
If China goes alone, they have to invest 100% in the project, Russia also can go alone too, in fact Russia can simply re-engine the Il-96 and modify it and go alone, but is that convenient?
The answer is not, first is time, if you want to make a B-777 you need the engine, military engines are difficult, but commercial aircraft engines are much more difficult?
Why? simple safety, if you notice the B-777 has been sold in larger numbers than the A-340 and il-96, Do you know why?
Simple the twin engine means higher simplicity, the 4 engines means lower thrust engines, but B-777 offers 2 engines with very safe qualities upon a very long range and low fuel consumption.
Can china make the engine? yes they can the question is when?, if it enters after several decades kaput, the aircraft becomes obsolete, so then you rely in foreign suppliers.
Market share.
A wide body is less demanded than a shorter range, smaller aircraft, so if you try to compete with Boeing, if you jet is obsolete you lose, airliners will choose right away Boeing, that is what happened in Russia.
Aeroflot did not use Il-96, they started using Airbus and Boeings, because the market demands those types of aircraft .
B-777 defeated the A-340 and Il-96, that is a reality, in fact even B-767 was defeat in sales by the B-777.
B-767 defeated the DC-10, L-1011, Il-86.
If you think everything is about the project you are wrong, aircraft are business, get them on time, get them with the best price per unit and operation costs then you get a winner.
Is not that Germany can go aloneor not or France can not go alone, they can go alone, they did it before 1970, go and google 1970s and 1960s airliners and you will find they did, There are VC-10s, Mercures, Fokkers, etc etc but the question is why compete in a very small business? the reality Airbus showed a larger company is better than many smaller companies making the same type of aircraft. why because these companies did not get bankrupt and they are still in business
COMAC and UAC know that, there is no room for each nation making the same type of aircraft and competing, in fact the Japanese have been really fool with MRJ by going alone, these days the best way to succeed in the aviation business is cooperate, Embraer is not Brazilian, it is Brazilian only in name, in reality is a mini Airbus, that has made them be the third largest manufacturer.
Nationalism is good for the masses, but not good for business, at least in commercial aviation