Pundit, I think you are somewhat right for this year reviewing the standard I have seen but 16A not 15.
However, every AIC premiership winner for the last 5 years would beat what the bottom 4 sides in GPS roll out and some years would have competed with some of the top schools. And we all know why the top GPS rugby schools flourish. For example, Southport has won half of its 8 outright premierships and all of their 3 shared premierships since 2000. Kudos to their meritorious first XVs of 1922, 1926, 1933 and 1938.
BBC won 100% of its outright premierships in 2020. But the 1954 monkey was a big one for the last 10 years, and expectations have been up there for a few years with the money spent.
Take away their recruitment "tools" and see what sort of First XVs you name. As painful as it might be for GPS supporters to read, TSS and BBC wouldn't make the top 3 in AIC because they are so top heavy with recruits but Nudgee would still be relatively strong because of its long heritage as a successful rugby school and a lot of next generation talent but certainly, wouldn't be as dominant without their usual intakes. Not so sure on Churchie (golden era in 50s and 60s) and State High (whose catchment is BIG) but I suspect Churchie's success of 2 outright and 2 shared premierships since 2000 only had them competing because of one thing.
The coaching has not always better either. Otherwise, QLD would have had more Super Rugby success in the last 20 years given the majority of players had GPS schooling backgrounds. But the GPS funnel into a Reds pathway development was broken and we have only seen an appropriate skills focused program in the last 5-6 years, now paying dividends with lots to come. There were also 5/6 GPS old boys in the Brumbies starting XV most weeks. Some of them are the best young talent in the country.
I am not against what the GPS schools do and know there is another thread for this discussion. They are definitely next level and good luck to them.