Complaining about the validity of grades is an easy path to deciding "I could do better than them if I actually tried" for me. I've regretted not trying, but I've never regretted working hard, regardless of the grade.
In the UK my eventual grades were decided by an impartial third party who didn't even know my name or sex, simply a number. Sure in this system I still could have done better if I'd tried harder, however I didn't try harder and I don't today, so really it is an apt measurement of my skills. Someone who is working their ass off at 15 is still going to be working it off at 50, I beat them without effort at 15 and logic dictates I'll likely still be beating them at 50 without effort.
One of my friends told me that the college he went to decided at some point that they think the SAT is stupid and they should stop using it as a basis for admissions. In order to get an idea of students' math performance, they would instead submit one (1) graded math test that they had taken in high school. Obviously, most students submitted the highest score they had achieved, which was usually the easiest test they had taken. Some students actually had the integrity to submit tests with mediocre scores but which nonetheless demonstrated significant understanding of the concepts. One poor professor had to review these tests, and decided admission based on them. He, quite properly, gave stronger recommendations for students in the second category, who submitted harder tests with lower scores. Politics ensued, and there was no happy ending.