Their downfall was earlier than that. Post WW2 everyone was looking to buy a new car (people kept their old one during the war because production was going to the war effort). The car companies had such demand they moved to a 'car salesman' sales structure to milk every customer as much as possible because demand was so much higher than production. They got hooked on the easy money and entrenched a lot of bad business practices/policies as a result.

GM for all intents and purposes died (remember we funded a whole new GM, a completely new business entity, during the 2008 financial crisis timeframe) and yet new GM just 'invested' 6 billion dollars in stock buybacks, millions in management bonuses while conducting employee layoffs. But they will have no problem coming and asking the government for billions 'to remain competitive' soon. F'm.