What makes you think that? It's on the public record that NBN Co were fairly well on-budget (after they'd upped the peak funding from $39bn to $44.1bn a few years earlier) up until to the 2013 election.

Of course, the media preached it was over budget, so that's what most people think, and Malcolm Turnbull was so suspicious that he commissioned a forensic review into their financials - which was very quietly released when it found "no material issues". There was also some evidence of the incoming NBN management choosing to keep quiet and shut down a cost reduction program that had successfully found efficiency gains and cost improvements ('Project Fox' which was leaked to the media).

All major fibre rollouts have tended to show that costs reduce over time - Chorus in NZ reported a 44% reduction in per-premises cost over their rollout so far, and Openreach in the UK have reported a 50% reduction from when they started.

Coupled with the fact that the Coalition's plan blew out by $15 billion, I think it would have actually been better to continue with the original plan and try to fix the problems. What they ended up doing was to not fix any of the existing problems, and try to 'fix' the things that were good about the NBN (the technology)...

This isn't counting that any upgrade to FTTN (which will be required in less than 10 years) would cost tens of billions of dollars more, or the much higher running cost of FTTN. In the end, the LNP really picked a dud and absolutely screwed up something that could have been fixed.