I also think it was silly to offer pensions in the first place since it is not possible to predict the future. People working for cities and other organizations offering pensions should be very concerned by recent events.
Every contract is formed in the shadow of its governing law.
Municipal bankruptcies have been a part of the law of the United States since 1937.
∴ no promises are being broken in a meaningful sense.
I think a big failing of government pension funds (and Social Security) is that there is always a tremendous temptation for politicans to borrow against them, and leave the repayment to someone who will be elected after they're long gone. The next person kicks the can down the road, and sooner or later you're Detroit.
With sane and responsible management I'm sure some pensions would fail anyway because there's always the chance of catastrophe. But I've seen enough pension problems that were the result of risky investing, poor forecasting, and irresponsible borrowing that I feel like most of the time the pension needn't have failed or gotten cut.