MEL definitely should get a rail link, but I do not see it coming. There are very good linked Airports (like FRA or CDG), some that have a rail connection but could improve (like MUC or LHR). The main problem in SYD is the price. The taxi is sometimes cheaper. As some rail links like to rip air travelers off to charge something extra for the airport service (yes MUC does it too and that rail-link is very slow).

I am lucky that I get picked up in MEL. My friends wait outside and I call them when I got my luggage. So no parking fee (no I only want to park. I do not want to buy the parking lot!)