You have to keep in mind that they were only around for two years, it's hard to make a serious impact in the culture in only two years, especially in a culture that doesn't traditionally take an interest in your product.
But to give you a better idea, their average crowds in 97 was 15,330, their biggest home crowd in 97 (and ever) was 27,435, which for the time is really good and shows heaps of potential for growth.
In 98 their crowds took a huge dip down to an average of 7,682 and their biggest attendance for the year was only 11,289, but every club saw huge drops in crowds at the time due to off field factors and every new clubs see a drop after the novelty factor wears off, it also didn't help that they were a hopeless team, but given time and some success they would have built themselves back up.
You're in a bubble, go out to areas that aren't traditionally Broncos areas and talk to them, sure you'll still get Broncos fans, and you'll get the ones that hate the Broncos, but most will simply be indifferent to the Broncos and that's the worst thing that can happen to any brand- indifference to it's existence...
Maybe if another team enters we can get the ones that are indifferent to the Broncos to fall in love with the new club and thus hate the Broncos because of their love for the new club, which would be a win, win, win for the new club, the NRL, and the Broncos cause it'd mean that they watch and take more of an interest in all of those groups content.