Rugby may well be the biggest and most important sport to the old boy community - particularly those aged 40 plus, but even a casual observer would note the change in demographics across all the GPS schools. Certainly this applies to some more than others, but the days of the anglo-Australian masses playing rugby, rowing or cricket have long gone for most if not all of the schools. The majority of boys no longer play these sports.
At SHS, SGS and NC, soccer is either equal or greater than rugby in terms of participation, Joeys have won 2 GPS soccer competitions since their last rugby triumph, Riverview have embraced Aussie Rules and Basketball has eclipsed cricket just about everywhere. When the old boys of the last 5-10 years and those that follow become the majority, most of them won't have played rugby, so the idea that rugby has some divine right to pre-eminence is false IMO.
The only thing that will keep the GPS rugby going is a competition where the maximum number of schools have a chance of winning (not each winning the same number of times - but every so often a talented year group comes through and wins a memorable victory). The sports that provide meaningful competition for all 8 schools will be the sports that will prosper into the future. This is the big danger for rugby, and alarm bells should be ringing for all true supporters of the game. If the GPS schools chose to discontinue or downgrade their involvement any of their sports, it would barely create a ripple as all of the other sports have a robust junior club system which is their primary source of development. Rugby is the only exception to this.
Some people should be very careful what they wish for.
EDIT:
The following sports have only been GPS sports since:
Soccer 1988, Cross Country 1988, Water Polo 1988, Basketball 1975, Tennis 1972, in the words of Bob Dylan "O the times they are a changing"