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).

> But even when a city says to unionized workers, you can keep your pension but new hires get moved to a 401k, the unions say no.

Well, yeah, because then 5 years down the road, when the city tries to shift pension contributions from the city to covered employees on the defined benefit plan, newer employees have no incentive to support the older employees and the employer can split the union. Which is why unions resist anything that gives different covered employees radically different contract interests.