How would you go about it?
I don't pretend to know enough to plan, that's why I speak of aspiration rather than targets or goals. In raising the issue, I hope to get the attention of someone knowledgeable, with the wit to think outside the box and the courage to think at the necessary scale.
Before making such assertions, you might like to study a little history. At that time, Australia's major industries were agricultural and therefore rural. In 1901 around 50% of the population lived outside major centres, either on the land or in small settlements. We were, in fact, far more "spread out".
I think you'll find that the population preceded the telephone by around a century. The network had a lot of catching-up to do, hence the overhead wires and party line mentioned in one of the links from the opening piece. Those were stop-gaps, just as satellite and fixed wireless are today.
Copper is limited by its service life. The network can't be any larger than what can be replaced within the service life of the technology. Even so, our forebears managed to get copper to some surprisingly remote locations. Places that I doubt we'd have the courage to attempt these days.
The service life of fibre is many times that of copper, so the network is potentially many times larger. If we're going to rely on satellites as more than stop-gaps then, given the exponential rise in demand and the short service life of a satellite, we're going to need an awful lot of them. <http://www.abc.net.au/news/2015-12-15/outback-nbn-internet-plan-won't-end-data-drought-lobbyist-says/7028818>
I reckon you've answered your own question. We would aspire to cover Australia with mobile telecommunications because its sparsely populated. The more remote the location, the more vital is communication; literally - lives have ended for lack of it. I have relatives travelling the outback right now; they have an EPIRB, but the cost of a sat. 'phone was beyond them (actually, I don't think they believed me when I told them that their mobiles wouldn't pick up a signal over most of their route).
If we're going to push fibre to the max, then why wouldn't we leverage that investment as backhaul for the mobile network?
The social good and business benefits are difficult to quantify, so accounting for and offsetting them against the costs of the network would be problematic. I've no doubt, however, that the potential benefits are substantial.
I'm guessing that you look at this from a short-term profit perspective. My long-term aspiration would be to break even, at best, and run at a loss if necessary. Running the entire network as a coordinated whole, I reckon the profitable parts could more than offset the others. If not, then it probably doesn't matter much.
When discussing the NBN, we tend to concentrate on Internet access. The fibre-to-the-premises (FttP) plan had other dimensions. It provided for both video and audio, outside the Internet. The long-term intent, as I understand it, was to transfer as much as possible of what is currently broadcast onto the fibre. That would potentially free up radio-frequency resources. Ending broadcasts is only practical where FttP is ubiquitous.
At present, valuable longer-wavelength radio resources are wasted on metropolitan broadcasting. There's substantial potential in those resources for mobile telecommunications covering remote and maritime areas. (Longer wavelengths propagate better over distance and around obstacles.)