I don't think that's going to eventuate though. Australia's FC system breeds a bowling attack that's largely adaptable to away Tests, especially in England, South Africa and the West Indies. Given the eclectic nature of their own home pitches, it's somewhat of a necessity anyway. What you're supposing is that Australia degenerates into something like the formerly Indian or latterly English model of cricket, while a better, more adaptable side adopts an FC model that's more conducive to what would usually be an Australian team makeup. So, in other words, the universe turns upside down.
What you're saying is hypothetically sound, but I think is fairly unlikely to happen. In light of that, I think the only conclusive way of determining a changeover is by defeat in a home series.
We have been blessed (cursed?) however, with two sides in 70/80s Windies and 90s/2000s Australia, that are good enough to consistently win away and support my theory of this 'changeover', in that they started to dominate in a way that happened
incidentally after they began to beat the previous best in their backyard. The competitive climate in Test cricket could well change in that there's no clear dominator after Australia eventually lose at home, and ergo no single 'world champion' to be clearly determined.
Which would **** on my argument rather emphatically.