You can still make it a defined-benefit plan. A defined benefit plan should be easy for the government to budget for---the benefits to be paid out are very easy to forecast. (And in essence equivalent to a bond, or perhaps annuity.)
"Budget" is the operative word there: Controlling the expenditure of tax dollars is one of the fundamental things elected officials do in a democracy. Thus current governments can't ever truly bind future ones.
Unless the money is in an account with your name on it, what you have is a political promise, not a contractual obligation or an asset.
