Broncos crowds ARE shit.
BUT its not really their fault; what you just said expressed the problem perfectly. They, in a much bigger market than any other team, dont have crowds or membership to match. This isnt because Sydney fans are better in some way, its because the broncos are already the biggest club in the league, they dont need to push themselves.
This is why the NRL needs another brisbane team. Imagine if the Brothers came in and, within 5 years, had more members and bigger crowds than the Broncos, who had had a 25 year head start. Broncos would be so embarrassed that they would really start to push membership and crowds to the limits of their potential.
The thing that brisbane really needs is competition.