In fact ADSL2+ speeds should be increasing as more and more people are offloaded onto NBN and the load on the system is lightened. Exactly the same thing happened when ADSL took over from dial-up. When all your neighbours moved onto ADSL2+ back in the day, you could actually get a workable/useable 56K/V.90 connection again when no-one else was using dial-up anymore.
At least that's what would be happening if there wasn't external artifical forces deliberately crippling the remaining ADSL customer connection speeds to force them onto NBN in frustration. They would argue that if they don't deliberately cripple the remaining ADSL customers, then they would never have a reason to want ot move because for them being the last 1% using 100% of the ADSL technology capacity all to themselves, the speed & reliability would be so good, they'd see no reason to get NBN at all.