For this to be a successful bit of infrastructure the government needs the NBN to reach a point where they are self sustaining in a reasonable amount of time. This means that the large ISPs had to sign contracts to move people away from ADSL. To combat this, the ISPs seem to be pushing their customers to move to cable, they are trying to keep customers on their old telephony infrastructure, and are holding out on negotiating some things with the NBN Co.
In essence, they are using their market power to push back on the NBN, to try get the most favourable contracts possible, while locking their customers into old infrastructure technology, which means the NBN may take a lot longer to become a viable project.
"...you'd still need to keep the copper running for non-subscribers"
There aren't supposed to be any non-subscribers, as all telephone, internet, and eventually television, is supposed to run over fiber.
"Seems like a reasonable trade-off if they are taking the savings and using the capital to accelerate the roll-out of fibre internet."
They aren't. They are doing this, if anything, to hamper the roll-out of the fiber. It's also good to remember, that this company (Telstra) was essentially given this infrastructure by the government, as it was previously Australia's telecom monopoly. Now they use that infrastructure as a weapon. The NBN Co (the people rolling out the fiber) are a completely separate entity.
"Even if you put a fibre-based POTS system in every exchange..."
You don't need this, as VoIP has been around for a fair while now, and many ISP's bundle their internet with VoIP. Most modern routers have ports for it, so it's not a problem. They aren't self powered, but most people have mobile phones which work fine most of the time, and many people have wireless home phones which also don't work when the power goes out. I'm sure that if this was a real problem, more solutions for self-powered would come up.