Then, rolling out an inferior tech (compared to fibre) that is only marginally better to current ADSL to the rest which is going to run over an aging copper network that will require billions of dollars of maintenance over the years? All while maintaining the current telco monopoly...
Really, their only selling point is the cost, but I haven't actually been able to find any evidence that the plan is actually costed - every reference to it being cheap is just based on rollouts in other countries with very different geography, population density and existing infrastructure. If they actually costed it, and then took running costs into account, I doubt it would actually be cheaper. And the current, superior NBN isn't even a cost to the taxpayer - it's an investment that will eventually pay itself back with a small profit.
I think the biggest thing though is that even if it were double the price and didn't pay itself back, the economic benefits of having fibre generally available to businesses and homes, as well as having a network which provides open access to suppliers (instead of having to go through the Telstra monopoly) would make it worthwhile.