Your system changes nothing as Chlorophile outlined.
If we want a real change in the education system, we have to change the education system. Whether it's exams getting easier, or students get better at exam technique, the end result is the same - children, manufactured in exam factories, who just plug in different numbers and words. Or memorize essays. Etc.
Scrap A-levels altogether. Move to a system, or qualification, which actually teaches students to think, and teaches them academic skills. A qualification where the skills aren't based on just rote learning and exam technique. And a qualification where a broad range of study is encouraged, not depth, so that 15 year old kids don't have to decide what field they want to enter (or more worryingly, lock themselves out of fields they don't even know about) for the rest of their lives.
We're undergoing a skills shortage at the worst possible time. Creating a workforce that has more flexibility (and real skills) will help that more than just changing the grading of the current system.