In an attempt at a serious answer, this is the one excuse I cant fathom.... Why would Adelaide NOT have a top flight club if United were relegated? In a true football pyramid the fans would still have a team to go and support.... it would just be in a second division trying to get promoted back up.
I mean 99% of the football world has countless examples of, often rather large cities/regions, where the football team spends decades outside of the top tier yet still manages to represent the region and strives to get promoted... Do people in Leeds for example just stop following football for decades at time? or people in Berlin? Do people really just follow a club in the ALegaue because they have been told it represents their whole state or region like Western United?