They retired. Boeing hasn't built a new plane in a very long time. Part of it is management, and part is regulatory issues. Yes management has consistently forced out people with knowledge, and replaced them with less experienced people. That happens in every industry, it's not always catastrophic.

The real problem is due to the regulatory environment. Yes those rules are important, but they've also effectively banned new aircraft from being built. There are now generations of engineers that are experienced in making a new aircraft look like a small tweak to an existing one. The perverse incentives created by the regulations changed Boeing from a company that built aircraft, to a company that just games regulation. A similar thing happened to the auto industry to a lesser extent.