I still believe that anyone can do 3 unit maths if they really want to.
Mathematics Extension 1 is a difficult course. 3 units is 180 h p.a. That's a lot of time. Choosing a 3-unit subject also limits your choice of other subjects, because it means that, if your school has a limit on the amount of units you can study, you have less units left to choose than if you chose Mathematics alone.
I disagree than "anyone can do 3 unit maths if they really want to" because, for a start, there are people who are born with genetic diseases such as Down's Syndrome, which limits their intellectual capacity and there is nothing in this day and age that can be done about it. Maybe in fifty years time we will have gene therapy that cures Down's Syndrome, but we don't yet.
Next, I assume by "anyone can do 3 unit maths" you meant that they can do it and do reasonably well at it, not just do it and flunk it. Learning in mathematics is progressive, so if a person has not been trying since Year 6 and has missed out on learning many key concepts, they are clearly not going to succeed in Mathematics Extension.
Finally, I would like to say that: General Mathematics is not "a sorry excuse for a subject". Studying it teaches students mathematical skills that have direct application to everyday activity, so it is a very useful subject for people who do not plan to continue studying mathematics outside of high school, or who wish to study accounting, economics, business or design.
Of course someone who studies Mathematics Extension 1 and Extension 2 or university mathematics as a major discipline would generally believe that General Mathematics is a sorry excuse for a subject. That doesn't mean it is though.