I get where you are coming from, I really do and I am a fan of a world class centralised stadium model but you also have to understand the communities you are talking about.
I have spent a good chunk of my life living in the shire and I can tell you the locals who are born and bred don't like leaving... for any reason. As far as they are concerned they are their own self sustained community and there is absolutely no reason to leave other than work nor would anyone in their right mind want to leave. For example, people living between Menai and Sylvania would find it just as easy to get to Kogarah as Woolooware for footy depending on traffic however one is in the shire and one is not and you just don't leave the shire.
That's just the culture in the area so I don't think the sharks moving to the CBD would ever work. If given financial incentives I could see them having 2-3 games a year at the SFS against the bigger drawing Sydney teams.
We did try that in 1997 during Super League and it flopped.
The first home game against Canberra at the SFS had three men and a dog in the stands for want of a better phrase.
I was supposed to go that night (was 12 years old at the time) but didn't as my dad was sick with the flu on the morning of the game.
As you stated above, people from the Shire generally don't leave and don't want to leave the area.
I've always said had I been able to get work in the area and not get lumped with travelling to the city or North Sydney for most of my working life I would've found leaving for SA harder than I did.