While I agree that the NRL shouldn't adopt promotion/relegation, this doesn't make any sense.
Reserved away support at English grounds, for instance, is 10% of the stadium max. In a 30k stadium, you're talking about a potential maximum of 3,000 away supporters. Hardly what you'd call 'large amounts'. It's much, much less than the potential crowds in the NRL and auskick, which have half the league based in one city, allowing basically any fan who wants to attend an away game the opportunity to do so..
Also, Brazil, Russia and China are all bigger than Australia in terms of area, and yet all three have promotion/relegation. So it's not as simple as saying that density is a factor. Tradition of PR is far more important factor than density and size.
Firstly you've wildly misinterpreted my point. . . I didn't say that P&R wouldn't work in Australia because of the size of the population and lack of population density in Australia, I said that if you could negate the other reasons why P&R would be a bad idea in the NRL (or any of the other competitions in the country), that P&R would still manifest differently in Australia then to how it does in Europe because of said lack of the size and density of the population.
In other words, P&R may still work in Australia but it wouldn't result in outcomes similar to the ones you see in Europe, so even if P&R was instituted in the NRL Tri_colours and other like minded people almost certainly wouldn't achieve the outcome that they are looking to achieve anyway.
But ignoring the fact that you misinterpreted me:
Reserved away support at English grounds is only 10% because in most cases if they made it much higher it'd impact on home support numbers (i.e. they wouldn't be able to sell as many season tickets and the such), and they have other concerns, e.g. trying to prevent hooliganism and the such, that we only rarely have to consider here in Australia, that smaller, more controllable, groups of away supporters make easier to deal with.
I have no doubt that if they raised that percentage of reserved away support that they'd have no problem selling the tickets in most cases. However with the acceptation of big games and many of the games in Sydney, the NRL would be lucky to crack 1.5k in away support at most games, and I'd be willing to bet that the NRL could only dream of drawing 10% away support to most clubs games.
There is no legitimate competition to soccer in the sports market in Brazil (though that is slowly changing), so they can do things without having to worry about competition in the same way that sports in other markets have to.
The vast majority of the clubs that compete in the Russian soccer competitions, and almost all of the ones that have competed in the Russian Premier league, are from European Russia, which, depending on how you define European Russia, is an area of about 10 mil km² ( about 3 mil km² larger than the size of Australia). However not only is European Russia home to about 110mil of the 140mil people in Russia, it is extremely densely populated along the west of European Russia it's self, and that densely populated part in the west of European Russia (an area running from roughly St. Petersburg to Sochi, which is roughly comparable to the size of the eastern seaboard of Australia) is where almost all of the clubs come from.
So yeah, you've misrepresented the situation that Soccer in Russia is in, and if Australia had a population and the density of population that Russia has in European Russia, and specifically in a band of land that is roughly the distance of Carins to Melbourne, then Australia would certainly be able to support P&R as well.
Finally, China is a whole other kettle of fish, but put simply, because of their political and economic systems most of the clubs in China aren't under the same commercial pressures that they are in other markets, but even if they were it's a similar situation to Russia where most of the club are from the densely populated part in the east of the country and not spread evenly across the country it's self.