Why not? Boeing had domestic competition in the airliner industry until the 90s. They still have competition in the military contracting side of their business for most classes of product. Whatever went wrong with Boeing due to protectionism preventing competition must have happened quite fast.
Airbus's other European competition disappeared at around the same time Boeing's US competition did. Further, the time is not how long it took for the problems to manifest but how long they took to be recognized. Boeing was not put under the microscope until long after the rot had taken hold. Airbus has yet to be put under the microscope.
