There's quite a few news stories out there about people that were refused warranty work because they couldn't prove they had performed the required maintenance, so if you want to risk that to save a couple hundred bucks on an oil change, well, how lucky do you feel... as Dirty Harry said.
Resale value could be affected if you don't have service records that show the car was looked after properly.
Remember that email you get every month from GM on your car's status - mileage, oil change status, tire pressure, systems status, etc., and who knows what else they can see on their end - you're not paranoid, Big Brother (the General) really is watching you
Sure, modern engines run cleaner and oils are a lot better than days gone by, that's why oil change intervals are much longer nowadays, and you could get used oil analysis done to try to extend oil change intervals, but I doubt that would be cheaper than following the recommendations in the manual. You can't tell the amount of additive depletion or wear metals in the oil by looking at the dipstick. Your engine, your wallet, your choice