Boeing's market is global, and it does not matter if you can fly within USA if flights are shut down everywhere else - in the coming year any USA domestic airline will be able to get cheap planes from other airlines instead of buying new ones from Boeing.
If global demand for flights is down even 20% then this means that the global demand for new planes is close to zero.
If global demand for flights is down even 20% then this means that the global demand for new planes is close to zero.