Sydney's most valuable asset in my opinion is the fact that it has suburban grounds. You go into a specific area, and they're mad for THEIR team.
The situation for the AFL in Melbourne is quite different, as the teams are dotted around the CBD, and clubs really don't have fortress suburbs like in the NRL. Etihad and the MCG are so accessible too, as Melbourne's public transport to the city, and the roads to the city make the CBD very easy to get to. Sydney's road network is rubbish, and ANZ is miles from the city, and SFS is miles and miles from places like Penrith of Campbelltown. Melbourne also benefits from the shape of the Bay essentially allows suburbs to radiate in almost every direction from the CBD, allowing the population to reside closer to the city than in Sydney where suburbs can really only cost west of the coast, making it so much more spread out.
The suburban derby adds so much to the NRL, and is something that would take away a lot from the game if lost. In a perfect world, we'd be able to afford to have high-quality boutique stadiums in the suburbs, with decent roads and public transport to all. But playing everything at ANZ isn't the answer.