It doesn't invalidate your point, but quantity of oil used was never the problem with plastic packaging - plastic is a minuscule part of oil demand, and doesn't get burned at that. The problem is and always has been the plastic entering the environment, where it doesn't biodegrade and causes all kinds of problems. That's why eliminating the very light plastic grocery bags is a win, even if the replacement bags are just heavier ones you have to pay for, and even if total plastic consumption increases - the heavy bags are less likely to blow away into the environment. It's also the reasoning behind cutting down on plastic straws - it's not that plastic straws represent a large quantity of plastic, they just represent a disproportionately large percentage of harm caused by plastic pollution.