Last thing we need is two teams in Western Sydney. One will never work let alone two. It is the part of Australia that likes RL the most and AF the least. Totally unsuitable demographics.
By the time the aerotropolis is up and running, GWS will not be in Western Sydney. They'll be squarely Central Sydney.
The idea behind the aerotropolis is creating a third city within Sydney. The Swans already represent the "Eastern Harbour City". The Giants are based out of the "Central River City", and the aerotropolis will be the centre of the "Western Parklands City".
The theory is to make it possible for most of the people in each city to get to their respective CBDs in 30 minutes, so it's advantageous to have stadiums in the centre of these cities.
As Johnny Bananas said, nobody is seriously considering a third Sydney team before 2050. By then, GWS would have had four decades to establish themselves (longer than the Swans had before GWS' entry) and the Central City will have nearly 2m people.
By then, the Western City will have a further 1.5m people (essentially an Adelaide without a team). And unless West Tigers make a strategic move, there'll be no major team based in the middle of that city.
I'm not saying there should definitely be a team based there, but a lot can happen in 30 years. It should at least be on the AFL's radar. Especially with much of the aerotropolis still a blank canvas. At a minimum, the AFL should be ensuring there's AFL infrastructure available in case they do decide to make a move.