So why would we want to go down that route if a much bigger country (actually 2 countries if you include Canada) with a massive population and stronger economies are not even there yet?
In theory, because FIFA bylaws say that promotion-relegation is the norm. The US is a "special case" for all sorts of reasons, one of which might sound familiar - because clubs are franchises which need security of tenure in order to invest.
The other factor is that there are a whole bunch of NSL die-hards in Australian football who won't rest until South Melbourne and/or Woollongong get their shot at the big time.