They don't need to create new brands like "Saturn". Toyota, Honda and Nissan succeeded in creating new premium brands but their situation was slightly different. (My jury's still out whether they actually needed to change the name).
'So GM felt the need for an import-fighter brand. Creating a new brand without a very clear differentiator is a very expensive undertaking. Usually ego driven, unnecessary and a waste of resources.
Detroit had some world-class sport/luxury brands in the 60's. Lincoln, Thunderbird, Mustang, Cadillac, Covette, Camaro, Imperial, Charger and Challenger come to mind. 'But they just could not resist t5he temptation to protitute these brands with cheap versions.
Basically that's all Detroit needs to do to compete with imports. Don't over produce your premium brands. If demand gets too strong raise the price, NOT production.