Strewth
You are asking the wrong question and because of that reached the wrong conclusion.
If soccer does become the main sport in private does not mean rugby will disappear. Nor will the investment in coaching etc, it may come down a little but it will never disappear.
Why is soccer becoming the sport of choice for many well the reasons are many the soccer mum, the fear of being hurt, recent reported cases of spinal injuries even if RL they carry through.
However arguably the two biggest reason is the changing demographic of the school population and the parents. Today many more Asian students with parents who do not share or are aware of rugby traditions. These people are far more open to soccer.
The second reason and again its simply grassroots management in that soccer does it better than any other sport. I had asked the teacher at Barker the key reason behind his statement and he shook his head and said its so F****g obvious and he said he had written to the ARU about it.
At school level especially high school rugby places the year 12 student and the final year team as the best. Soccer does it in years 8 & 9 therefore getting in early and letting students study in years 11 & 12.
Its called the Bill Turner Cup and its Australians biggest school tournament.
Have a look at this site and more to the point look at the private schools heavily involved. Especially look at the top 32 schools in the finals its on the front page of their web site. Its also worth having a look at this site its history, development and that it has captured the interest of many teachers and their parents. Because of the U 15 part, students studies are not interfered with in the "ATR" mark years as students prepare for HSC exams. However the concept of a 400 + knock out competition means the winning school or even top 32 school is considered an achievement. As I said soccer does grassroots things so much better than any other code.
http://www.billturnersoccer.com.au
In the 2015 results a number of private schools made the final 32 in both boys and girls.