Since when is an uptake of around 50% for anything, 'less-than-enthusiastic'? If I was selling a new product into a market that has active competition, I'd be delighted with a ~50% uptake.
Those that do sign up for an NBN connection mostly aren't buying faster connections.
Unless we know the before-and-after speeds, this statement has no basis in fact. Even not knowing specific customer speeds, it should be noted that the rollout started in places that had slower available speeds initially. Many of the areas first serviced had, at best, standard ADSL1, 8Mbps. And this is very dependent on distance. The vast majority of people 'on' 8Mbps ADSL1 were getting speeds far, far lower than 8Mbps. Therefore a guaranteed 12/1 would be at least 50%, if not much better, than they would have had before the NBN rolled in, therefore 12/1 is 'a faster speed' than many had beforehand.
The fastest speeds available to most of the rest of the country was ADSL2+, 24Mbps/1Mbps (up to 2Mbps up if using Annex M, but that reduces down speed), excluding those lucky few, a very tiny %, who could get cable or VDSL. And again, most people 'on' ADSL2+ won't get anywhere near 24Mbps. I'm on ADSL2+, the connection is very unreliable, mostly sitting at around 14Mbps, sometimes dropping as low as 3Mbps for periods of a few days (probably due to bad cables). And I won't get NBN in my area - a suburb of Canberra, the capital city, for another 2 years (at the earliest).
Therefore, a guaranteed 12/1 (the lowest plan in the graph, ~33%) would improve my service to a stable, guaranteed speed.
And the 25/5 plan, a guaranteed speed as I understand it, is better than any ADSL2+ theoretical capability, and significantly better than any 'realistic' delivered capability. I'd be delighted with 25/5, and for my parents, for example, I wouldn't recommend anything higher than 25/5, as that would be more than sufficient for their use-pattern, 12/1 would do them now, but 25/5 would be better for opening them up to more capability.
As people who were previously on ADSL1/2 get experience with the higher, stable, speeds of NBN, and as more services become available from the internet (Netflix only recently came to Australia for example), and as long as it remains affordable, more people will migrate to higher speeds.