At present C919 has received about 850 orders and options. These orders and options would take 10 to 15 years to complete.
China would need to bring in around 6,100 new aircraft in the C919 class according to Airbus, which mean there is a need to bring in around 4,000 to 5,000 Airbus and Boeing single-aisle jets even if COMAC is turning out C919 in full prooduction capacity for the next 20 years.
Which mean China can concentrate on domestic marker and put aside effect of US not issuing FAA certification for 15 to 20 years. And also the huge domestic demand for new aircraft is a big leverage for China against Boeing if US decides to ban China from buying engines and other components for C919 jets.
According to Airbus:-
According to Airbus’ 2018-2037 Global Market Forecast, new deliveries of passenger and freight aircraft for China will be more than 7,400 over the next 20 years. In the Small segment, typically covering the space where most of today’s single-aisle aircraft compete, there is a requirement for 6,180 new aircraft; in the Medium segment, for missions requiring additional capacity and range flexibility, represented by smaller widebodies and longer-range single-aisle aircraft, Airbus forecasts demand for 870 passenger and freight aircraft. For additional capacity and range flexibility, in the Large segment where most A350s are present today, there is a need for 240 aircraft. In the Extra-Large segment, typically reflecting high capacity and long range missions by the largest aircraft types including the A350-1000 and the A380, Airbus forecasts demand for 130 aircraft.