The choice is raising taxes, reducing spending elsewhere, or demanding workers give up the pensions they were promised. Somehow I doubt anyone is going to agree to pay more in taxes to cover pensions in their state or city. After all, why should the government keep its promises (especially if it means we have to pay for it)?
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. See San Jose as an example
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.