Okay, I agree with you here.
However, while I can see the point in your argument, the grade 9 boundaries aren't exactly that high for a grade of its supposed worth. I would not consider AQA biology too much of an outlier for the grade 9 boundary. For example, AQA geography (grade 9 boundary 183/252 marks - *72.6%), physics (grade 9 boundary 135/200 marks - 67.5%), route DA for history (grade 9 boundary: 107/168 marks - *63.7%), combined science (trilogy grade 9 boundary: 289/420 marks - *68.8%; synergy grade 9 boundary: 248/400 marks - 62%). Let's not just attack AQA too much either, Edexcel's combined science course wasn't too high (grade 9 boundary: 260/360 marks - *72.2%), computer science (113/160 marks - *70.6%) and religious studies B specification (for papers 1A and 3C, grade 9 boundary - *65.2%).
Now, obviously, I'm cherry picking my data a bit here, but seeing as how the grade 9 is so 'great', and requires a vast majority of the content to be answered, those boundaries don't do it much justice. A student may aim high, but if they don't score as highly as they aim, it doesn't matter, because they are allowed to get away with so many errors. A grade that is meant to represent such a high level of achievement should NOT have boundaries like that. In reality, they should be around 95%, so that a student cannot get away with so many errors. Just because a student aims highly, does not mean that they won't make careless errors. A grade 9 boundary should not allow so many careless errors to be made to the point that the percentage of marks required to achieve it drops below 80%. The highest boundary that I actually saw was the AQA XB option for the religious studies B specification (grade 9 boundary: 185/204 marks - *90.7% marks)
*For argument's sake, I just rounded the non-integer percentages to three significant figures.
A valid argument, but once again, just because they aim high and try and learn the content and understand it, does not mean that careless errors won't be made under pressure in the exam. Now, a few careless errors can be made, I don't mind. But if a student makes man careless errors, and ends up with 68% or 73%, they should not have access to the grade 9. This is a flaw in the grade boundary system, in my opinion.
A grade 9 requires a student to aim for top marks, but not necessarily achieve it. They may assume that the boundary will be high, but they can still get away with a pretty average mark. I know a friend who was aiming for around 90% in physics, but got a grade 9 with something like 70%. He might have aimed high, but he absolutely flopped relative to his expectations. He should not have had a grade 9 with 70%. In the end though, this is just my opinion. Your arguments are pretty valid, and I can see from your perspective.
I just feel that grade 9 just hasn't lived up to its expectations just yet. We should wait a bit longer. After a few more years, I'm sure it will finally become what it was meant to be. Right now though, a grade 9 just requires a student to aim high, not actually score high, seeing as they can get away with being average.