When an industry expands quickly more people have to rise to the challenge of becoming new good bosses because the demand for good bosses outstrips existing supply. In addition, old good bosses retire rich mid-career, depleting the stock. In this situation you may find yourself thinking there are no good bosses at all, and you're the next King Arthur. Which you might be - someone has to be.
When an industry grows slowly the stream of retiring good bosses is matched by the ascenders, maintaining an environment saturated in good bosses.
When an industry stagnates the good bosses retire and their places are taken by the peace-time bosses, people who learned to follow in footsteps of the elders but never learned why. They will mostly be bad bosses due to lack of experience in making errors and for lack of selection pressure that would push them against their own errors to select the best.