I think the only thing that you are missing is that the average roll for an attribute score of 1 is still always higher than any equivalent roll with an attribute score of 0. Otherwise, you are spot on. You can find more detailed results of the average rolls here: Average Roll Breakdown by Attribute Dice
This would obviously change if you’d allow more than 1 advantage on the d20, but that’s why the attribute score of 0 has a cap, while no others have.