You can't retrospectively means test a state pension after so many have paid into for most of their working lives.
You can say that from x date, perhaps with 10/15 years of their working lives left, that changes will be made, to allow those affected to make the necessary provisions. But that would have to be phased in based on their projected pension shortfall.
As for public employees not contributing to their pensions, this was only ever partly true. I knew when I joined local Govt. from Central Govt back in 1993 that I would ever thus be contributing to my pension and the higher I went the larger that contribution was. But when I left Central Govt (which I never wanted to do and only did so as my Dept. was being subsumed into another and I was never prepared to work for the other lot), my salary went up by 4k per annum, but nearly 50% went to my new pension conts.
And although I was moving on to effectively, a lesser job, but one with great potential, the 4k increase really demonstrated how, as civil servants, you knew there was a trade off between your lower salary and pension, irrespective if that is formally correct, it's why most were prepared to make do with that - and a sense of pride and responsibility, which certainly applied in my line of work. I could have easily done a desk job, rather than working as and when required anywhere the job took me, as I did for many years.
And for what it's worth, all civil servants now make some contribution to their pensions. As far as I am aware, no one gets a free ride anymore and other changes have been made as well - final salary pay out replaced by average salary pay out, working longer and so on.
And as an edited addendum, has anyone actually checked what the average public sector pension now is? A few years ago it was 8k per annum.