The general problem with GM (& Ford & the Chrysler part of Diamler-Chrysler) is that they make lousy cars.
Usually, when it comes to quality & such, making a good product doesn't cost much more than making a bad one, and in the case of cars the price of steel is pretty much the same. A lot of high quality 'foreign' cars are made in the US or Canada, so its not the workers or the wages.
Case in point: the model year of the last American (made in Canada) vehicle I owned had had more recalls than Toyota had had on all models as far as the records went back 20 years or so.
So setting aside lousy handling, uncomfortable drivers' position, and bottom of the barrel reliability, why would I even consider buying another 'domestic', except at a hugely discounted price? Actually, since I can afford it, why would I put up with the aggravation.
GM and the other could chose to make good, reliable, vehicles. They don't, as a matter of corporate policy.
Until they learn how the market now works, they are on a steady slide into oblivion.