> It may not seem fair to change the rules on public employees, but effectively the rules have changed for private employees

It's possible this has happened some places, but not at any major companies I know about. Companies that have phased out defined-benefit pensions have generally not done so retroactively, but only closed them to new hires. Phasing them out retroactively would be a breach of contract, since the employee performed work for you under the expectation that you'd honor the employment contract they signed, including the pension terms. Pension is a kind of remuneration, so you can't decide not to pay it after receiving the work.

Much easier legally to simply stop making those pension promises to new employees, but not breach the ones you already made.