There is no doubt in my mind that perception of the value of the Premiership versus the Championship has changed over the years to align with global football culture. Everyone I know sees the Premiership as the prize and the Championship simply as an end of season knockout cup, there to make money for APL. That's my take on declining finals attendances. The GF is different because it pulls in the event goers who would not normally go to an A League game anyway.
I'm an old fart now but played state div 1 when young, and it was then very much the Champions were the table toppers and then there was a cup comp for all comers. I have always found the Finals Series thing bizarre, because it is not the football culture I grew up with, and I've always been irked by the fact that it is yet another cultural cringe thing we have plastered on from the hick sports of AFL and NRL. I guess it gives the season a focal point, but if we were competitive in the ACL, I doubt that we would need that. AND if there was serious prize money for the Premiers.