First fo all, GM and Ford ain't going away anytime soon, they will struggle on for at least another decade or so through government money, mass layoffs, deep discounts, mergers and liquidations.
In the unlikely event that they do go away, don't worry, there is always Kia, Hyundai and Mitsubishi. And if the Chinese car makers manage to pass US safety regulations, we might be driving poorly made rip-offs of Japanese brands pretty soon (They are so blatant in pirating Japanese design it is downright shameless).