Word I got from a Boeing employee is that Boeing didn’t watch out for career progression of their junior people. They subcontracted out a number of short term projects for a couple of decades which let them get by with fewer employees but created a huge experience gap between the boomers who are now retiring and the more junior engineering people they now have to hire. And since they are the only remaining US airliner company they can’t exactly fill in that experience gap - at least not without headhunting Airbus engineers.