That would cause more trouble I'm afraid. The system itself is as fair as it will ever go in my opinion. It is hard to write one exam for a variety of schools at different stages of the year because schools set topics in different order and learn things differently. Our internal/external system is as fair as it will get.
If we had to have one system to cater all (like we do right now)
Is it fair to lower the quality of a course in order to give people a fair go?
Why does a fair go have to be a priority?
Of course like you have said Spiral, they could make 3U (Non Math) subjects. But look at the school trials for A LOT of the top selective schools. They are all pretty easy. The only respectable papers I have seen are Sydney Grammar ones and the odd one from other schools. So in the end if there are courses that cater to higher-end students, in the end teachers will abuse this and make easy exams for it.