the argument is between defined benefit vs defined contribution plans. The former are unsustainable as evidenced by illinois’ ballooning deficit, while the latter are somewhat variable in return.
I don't think it is clear at all that this is inevitable.
(It certainly seems likely that they will be underfunded; why not pay for services with promises that someone else has to fulfill)
Add to that fact that in the US, taxpayer funded pensions have NO rules (they're specifically exempt from laws regarding defined benefit pension funding), it's quite easy to see politicians giving away deferred benefits to government worker unions in exchange for votes today. The only person this affects (future taxpayers in 20 to 30 years), are mostly not at the table. And not enough people pay attention to local elections or understand the numbers enough to stop this.