Redcliffe seems to be the back up, It held a few events but only holds 12k.
What for the events where 20k is expected?
I think you’re applying the conditions of the Sydney market and naturally assuming that those conditions hold in other markets. Sydney has nine league sides, two fumbleball sides, three a-league sides and a union side. That’s way too many sides in a market like Sydney (there really is only room for just over half that) but putting that aside for one moment, it is likely that 20k is quite a reasonable expectation for most events, so there is a need for that type of stadium.
As Canard mentioned earlier Brisbane is a completely different scenario and market to Sydney. In Brisbane you have two NRL sides, an AFL side and a union side. What events would there be where 20k is the max expected attendance? They are not going to be NRLW games or QLD Cup games. Maybe A-League although Roar fans seemed happy with Redcliffe. Union maybe but who cares about them.
Essentially you are asking the QLD State government to build a stadium for League (predominantly) when you have the best rectangular stadium in the country based on the few times that you get less than 20k. What’s the point? If the Dolphins can’t get at least 20k average attendance in a city of more then two million people when they are only the second team in that city then they shouldn’t have been put in the competition.