I think while it's true(ish) there is still a huge population of short termers (not tourists), young tradies and people living there for the experience but don't necessarily stay forever. Also, like the Sunshine Coast, a lot of the residents have moved there as opposed to grew up there. That means they already have a team before they arrive. I think demographics like this have a big impact.I think that you could say this 10-15 years ago but I don't think so these days, the permanent population of the coast should be more than enough to sustain teams. The demographic angle is an interesting one, not sure on the age makeup of the coast. I certainly think poor or difficult access to stadiums certainly hurts them as does the lack of density on the coast. Neither of the stadiums are terribly easy to get to via public transport and neither have easy parking. I tend to think they've tried to go down the Australian way of stadium access with Public transport, but the GC isn't structured well enough that way. I tend to think they need to go down the American route of providing a lot of parking to make it easy for people to get to the game.
I remember working on the Sunny Coast, albeit 15 years ago, every second customer was a Victorian. I suspect the Gold Coast has more 'immigrants'.