Prepaid is the only way to do pensions.

If you don't pay pension obligations as you incur them, you're going to run into trouble if there's any biz contraction. And, if it's a long-lived organization, you don't actually reduce your cash-outflow by not paying when you incur.

Run a scenario where you under pay by X%. When folks start collecting, you have to come up with the difference out of then-current revenues. Steady-state with stable revenues, you end up paying exactly the same amount, but you've got debt. Now cut the revenue (and current expenses) by 25% because the world changed. Your revenue can't pay the debt.