Not sure it is as simple as that. The MCG has the biggest crowd capacity in the country (there were 100,013 at the GF apparently). None of the other regular footy stadiums hold more than 55,000 IIRC. So unless some other city builds a stadium with an oval arena with a capacity for 90,000+ fans, there will never be an economic case for holding it anywhere else. Even in an era when the TV rights run the game, not even the AFL are silly enough to pass up the income from 45,000 extra GF tickets (ie massive $$s) ...