Two points really - firstly, many public sector employees are very poorly paid (I think the average is about £15K but some are paid as little as £12K) and many, particularly women, are part time.
Secondly, the rules are changing anyway. New entrants won't get pensions based on years of service x final salary. It will be years of service x average salary over final (5?) years.
Given that many are low paid and part time and don't have many years of service (women returning to work as an example), the pension won't be that large. I had 20 years' service and was a senior manager but my pension is only about £4K a year. Hardly gold-plated and I struggled to pay 6% of my salary into the scheme.
So I think many of the lower paid workers will simply withdraw from the scheme and that will mean that the tax-payer will have to pick up the cost of pension credits etc in the same way as the tax payer does for private sector workers who don't / can't pay into a private scheme.