Consumers dont need more real bandwidth
It only takes 1.5 megabits to run Netflix (I know, I still have a shitty ADSL line). So 12.5 mbs is huge for most households. Given 25mbs costs the same, people will go for that, but very few are going to pay more.
Backhaul is a different matter, and maybe your 25mbs line only actually gets 5mbs in peak times. But the NBN does not want to admit to this issue, let alone charge for it.
And most (60%?) consumers were perfectly happy with their fast ADSL or cable systems which let them watch Netflix just fine. So it is politically impossible to charge them more for what they already had.
So, it is the tax payer that will be left wit the lemon.
This is where Turnbull lost all his credibility IMHO. He inherited political gold from Labor's NBN, but never capitalized on it when he was minister.