Compare the Korean cars of the 1980s with those made today.
There is a learning curve, but I highly doubt the world's major airlines nor their customers care to operate or take a ride on anything that sits upon such a curve.
If countries start buying Russian and Chinese aircraft in significant numbers because they have no alternative, this could be the catalyst that kicks off their aerospace industries.
That's true enough. Perhaps they could follow the Airbus model and practically give the early copies away, although they would need to have a quality bit of kit to peddle in the first place.
West Coast makes a good point regarding Russia and China. Witness their continued vetoes on the UN Security Council when matters far more grave than the sale of commercial transports were under consideration.
I agree, being worthy of trust in any bilateral or multilateral enterprise an issue, something the UK, EU and the US have in spades, although the current US president appears to be doing his best to erode some of that trust that has been steadily built up over decades.