You cannot change pension rules (or tax rules) retrospectively. Just imagine trying to administer it...
When in the private sector I paid the maximum possible from when I started work aged 21 into my DC pensions; now I have rolled them all up into one SIPP and I manage this for growth, as I still have 15+ years until I hope to draw from it.
Now in the public sector I have a career average pension, calculated annually, which to my mind is absurdly generous - however I pay a relatively large % in (but still get vastly more out in return as and when I come to claim it). But I get paid about 30% less in public than I would in private, for doing the same job.
All pensions are taxed based on the income tax rules prevailing at the time. Some of the benefits e.g. winter fuel, free TV license could be removed for those earning over £x (or else an equivalent tax charge applied through HMRC based on ages and thresholds).
However, to penalise someone (other than through normal income tax thresholds) because they have made the decision to prioritise pension contributions over and above other financial decisions is ridiculous.
Those 'old people' could be donating their 'excess income' to charity, their children / grandchildren, the local dogs' home etc etc (gifts out of income are entirely allowable and not taken into account for IHT or care purposes). They could also be saving that 'excess income' against their future care needs, to avoid this cost being paid for by the government.
Everyone should have a basic financial education at school and continuing to ensure that they actually understand how pensions work and the different types available, in addition to the range of state benefits, which are completely different.