Cops are one of the offenders, but make no mistake, teachers' unions are up there as well. There's no one particularly bad abuser of the pension systems; the system itself was set up by either innumerate or corrupt individuals. Or perhaps both.
If I was a pension rep or director for the cops, teachers, firefighters, or whatever, best believe I would maximize my members' benefits to the maximum extent the law and guidelines allowed. That's really all that is happening with these unions. Sure, there is some corruption going on (when isn't there in any large organization, much less government agencies) that isn't helping matters, and there are idiots that are making things worse. But these issues are in the seriously tiny minority of the real problem: People who could not do math properly or were corrupt from the get-go set up an insane pyramid scheme built on ridiculous population growth and capital appreciation models, and no one wanted to stop the music for decades on end.
The most likely scenario by a WIDE margin is that pensioners are wiped out or forcibly restructured in a court of law, getting a small reduced fraction of their promised ("guaranteed") benefit. It just is. In America, we don't fix gradual issues - there is simply no incentive to do so. We ignore them until they blow up and then we do something about it. This sounds stupid, but it's perfectly rational on an individual level.