GM could have cut GMC and, in the long term, I doubt many people would worry about it.
But, unlike Pontiac, I get the impression that most GMC products have a significantly higher transaction price than the average Pontiac did.
Pontiac may have had the volume over GMC at the end of the model year, but when it came down to the money, I very much doubt many people would have paid more for Pontiac’s even if they felt they were getting so much more.
Now, even though Chevrolet can do the work of GMC fairly easily, GMC is simply too profitable to throw away. If people want a glitzier truck or SUV, GMC will do it for them.
Pity the same couldn’t be said of Pontiac. Affordable horsepower did move cars off the lot; not so much nowadays, and it isn’t really GM’s fault; it’s more of a result of the times.