Here's a question - will airlines start selling "connecting flights" that fly into one airport and out of the other?
Seems nuts, but it happens in Tokyo, with some major carriers selling connecting flights involving the traveller getting themselves between Haneda and Narita. Even in Japan with very good ground transport, this is a pain. I think the official MCT for these itineraries is something like 3h30
Now imagine a foreign traveller flying in from say Europe, with an onward connection to Pacific islands or something, having to change airports at Sydney peak hour!