I blame Frederic Donner who became CEO (1958-'67) after Harlow Curtice retired.
Donner was a lot like Trump. He demanded total loyalty, or he showed you the door. This resulted in a culture of yes men, and the fact that Donner was a bean counter and not an engineer led to GM's down fall (an opinion shared by John DeLorean who was actually an executive at GM and saw it all).
Donner was correct in that buying parts is cheaper when applied to the bottom line than making parts, but you not only loose control of quality you loose capacity to make things. This was the start of the decline of GM and all of America's heavy manufacturing.
Bean counters should never be let out of the accounting department, and should only have an advisory capacity, not management. They are just a bunch of historians who can document the decline of your company.