I didn't say that a second or third Brisbane team for that matter would make AFL fans retreat or that it would even shrink the Lions' fan base. Not at all. But it absolutely makes any distant future hopes the AFL had of having two Brisbane teams a much tougher prospect and it also makes any further growth for the Lions difficult. There aren't that many AFL fans in Brisbane and with two Rugby League teams now, the casual sports fan who's preference leans towards RL but will go to the AFL if nothing else is on in town, now have a game every week to attend if they choose to and don't have to resort to the Lions.
So it is going to work like sunlight on a vampire and burn away all the people whom are naturally inclined to take an interest in Aussie Rules. . .
Those people aren't going to cease to exist because of peer pressure and airy-fairy bullshit about Brisbane being a "RL city" anymore than metalheads and country music fans are going to cease to exist because there's a greater variety of pop stars on offer at any one time.
Yes you can generalise about a population group, but preference is subjective, and in a population as large as Brisbane's there will always be sizeable groups whom prefer alternatives to the mainstream culture. It just is what it is.
The AFL is also a relatively new product to Brisbane, only having been around for about a generation and a half (arguably less than that). The mere fact that it's an option at all will probably lead to it taking a larger share of the market within younger generations, and it's impossible to truly know how the culture will adapt over time. For all we know RL in Brisbane could be some mismanagement and a bit of bad luck away from going into a steep decline that Aussie Rules, and/or other sports, could capitalise on.
Nothing is too big to fail so to speak, and if Baseball can go from 'America's favourite pastime' to a distant second behind Football with a couple generations then any sport can. Most would argue that it's been behind Basketball for at least a couple decades now as well, but I digress.