which sounds great in theory. But in reality very few cities in the world have a ready made athletics stadium of 60-80k capacity. Most need to build new or adapt an exiting large stadium.
Brisbane knew it didnt have a stadium suitable to host the main event. It was always going to need either a new stadium or substantial money spending making one fit for purpose.
Ironically, due to AFL, Australia is the only country in the world where you can justify building a large oval stadium for the Olympics as at least it will get used afterwards without major works needed. No other country has oval sports needing that level of stadia so they will always be either left with a white elephant or a substantial bill to turn it into a rectangular stadium for AF or soccer.
why do they even need 80k athletics stadiums anymore
Most of the money would come from TV so having 40 - 60k would be enough and converting a stadium to a rectangular ground afterwards works. Look at Manchester City with a new stadium after the 2002
In London they had the crazy idea that the main stadium would host 50k athletics post the games - but ended up being a football stadium - and now is going to be made rectangular
If Brisbane ends up with a brand new athletics tracks and improved facilities for 25k people with also a good quality warm up track - that would be great for that sport
Similarly the other big sport in Brisbane is RL which would make good use of an improved Suncorp