Assuming that you are building your premium brand as well as you can build a car, that means that you are intentionally building your budget line at a lower quality. One some things quality costs (more metal, tighter specs with more parts thrown out), but other things cost nothing or so little it doesn't matter. GM was the worst because then had to make a Cadillac better than an Oldsmobile, which was better than a Buick, which was better than a Pontiac, which was better than a Chevy. Throw in Hummer, Saturn and a few other brands in that stack and you either don't have room for differentiation or you have to make your Chevy really, really crappy.
My bet on the next car line to disappear is GMC. Its sole reason for existance was to allow Cadillac, Olds, Buick and Pontiac dealers to sell rebanded Chevy trucks. Just dump GMC and make all the trucks Chevies (or dump Chevy trucks and make them all GMC).