Objectively speaking, COMAC's C919 production is not able to fullfill all of China's need for passenger jets in the next 20 years. China will need around 7,000 new passenger jets, therefore there is a fair share for COMAC, Airbus and Boeing.
However, how much Boeing will receive orders from China will depend on US's trade, foreign and military policies towards China.
AT present, China-US relationship is at the lowest since the Cold war, Boeing is a victim because of Trump and Biden. Boeing will lose the 737 class orders to Airbus but they still can expect good sales from 777 and 787 if Biden is not stupid enough to make China-US relationship worst.
At the most optimistic situation where COMAC can made an average of 150 C919 per year for the next 20 years, they could only roll out 3,000 aircraft, which is still 4,000 short of the 7,000 aircraft China needs.
Boeing's fate will depend on US's attitute towards China. And we are not expecting the relationships will get better in the next 10 years, thus Airbus will be gaining at the expense of Boeing if they play the game nicely.
And fo a simple reason that China is a huge market for airliners, COMAC will grow at rapid rates to become major global player after expanding their production facilities as decades pass.