Except the production price for foreign makers in China is on the rise. The Main driver for making in the PRC these days is selling to the PRC. if you have a maker that pushes them out of the Chinese markets they will take a blow but then just consolidate. Buisness is Buisness, If they don't want your wares don't sell them.
Yet at it's route one factor your failing to consider is that the growth capacity of China's civil air is likely to be large enough that Comac and the big two will be able to compete on a even standing for some time. It will take years for Comac to expand it's production rate to meet demand that means Boeing and AB have time. And the chinese market is not the only Market. the Rest of Asia, Europe Oceania, and Americas. AB and Boeing have establish supply and maintenance chains meaning they can offer lower service costs then a startup. Also not everyone is willing to buy Chinese in this case.
So in the End Comac has a lot of work to do if it really aims to put Boeing and AB out of business. Yet Comac can go places where AB or Boeing are unlikely to meaning they have growth potential as well . Africa, Iran and Cuba, North Korea and Eritrea Places where Boeing is or is likely to be bared by Sanction or economics but where the PRC has no blocks.
Yet it all depends on that major Critical Factor of the Future, If this is infact as good a Aircraft as it looks then they can pull it off perhaps we will see the follow ons . If this proves to be like the Comac ARJ then it might just end up with a hand full or orders and a debacle.