Personally I think the whole issue laughable and is just this historians way to drum up speeking fees and book sales by expoloiting the current anti-american sentiment abroad. His 20% chance in 50 years is patently ludicrous unless as some of you have said there is some global economic or political catastrophe, and noone can predict those sorts of things. Even if it does who's to say Australia will need us? As the nation more hevily dependant on technology and the current economic status quo perhaps we would be seeking to become part of Australia.
Either possibility is just as likely, as in neither will happen.