For COMAC to satisfy just the domestic market in the next 20 years, it needs to build 450 aircraft per year. It is hoping to reach 150 C919s annually in 2028. Obviously, a lot of that demand will be filled by Airbus (many from Tianjin.)
But it is really a reflection of this market. COMAC doesn't have to export one plane to be successful. In fact, it doesn't need to take over the whole domestic market (and probably can't in next two decades) to be successful.
But like in autos, once China captures a large percentage (I say around 33-40%) of the domestic market its aircraft will be massively competitive in exports too because of economy of scale and world-class competition -- from the Tianjin Airbus plant -- just like domestic cars were honed by competition from JVs:
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