Well, what exactly do you expect? Unions do not like having some members receive wildly different benefits from others because that breaks their unity. Gradual phase-ins are needed if you are going to move to a defined-contribution plan; see e.g. the introduction of 401k plans for NYC subway workers, which has been slow and mostly agreeable to the union.

The other thing to keep in mind is that the problem is not merely that future pension obligations for new hires might not be funded. In some cases the problem is immediate or very near-term -- pension funds have become so depleted in some states and cities that retirees may not receive their checks, and in some cases people have been forced to accept less than they were promised (see the article for examples).