Also the state pension is very different from a private pension, insofar as you don't actually invest or sign any contractual agreement.
Much as people like to say that they 'paid into' their pensions for however long, they didn't. What they did was to pay the taxes that they were legally obliged to pay for however many years and then, upon attaining a certain government-prescribed age (which can change over time), and maybe with a requirement for having paid NI contributions (or being otherwise credited with having done so) for a certain amount of time, they then qualify for the benefit which is referred to as the state pension - regardless of whether they decide to stop working (or go part-time) or not. Like it or not, somebody receiving the state pension is a benefits claimant.
Just like if you claim out-of-work benefits, after having been in work beforehand for many years - you may quite legitimately reason that you've paid in to the system for years and so reconcile that with it being 'morally fair' that you having paid in when you could and now are being paid out by the system when you need it; however it would be absurd to claim that you 'paid into your unemployment benefits' (or maternity pay, or SSP), as though it were a scheme with a built-in entitlement. That's just not how society works.
My parents both died well before they reached state pension age, so in spite of their having paid taxes and NI contributions for years, they never received any state pension, as they never fulfilled all of the requirements to qualify for that benefit from the state.
By stark contrast, they had also paid into a private pension, which they never lived long enough to claim under the age-related terms; BUT as it was their money, that they had paid in to a scheme that was specifically set up and ringfenced for them, my DSis and I eventually received that money - because it was effectively their savings.
The government wasn't involved (other than in regulating the private scheme), so it wasn't a state benefit, as is the state pension. That scheme was exclusive to them: nobody else had paid into it and nobody other than them (or their heirs) could benefit from the capital that it earned.
Just like if Reeves had succeeded in her plan to take benefits away from disabled people who couldn't work, however morally justified, none of them could have claimed that she had taken away money that they had 'paid in to' and were thus 'entitled' to, even if they'd paid £30m in tax before becoming disabled.
Governments change goalposts - and they have a right to do so. I'm amazed that people ever choose to rely on them to 'do the right thing', to be honest.