Well also, 20 years is less time then you think. For a system of this magnitude, deploying the replacement could easily take 5 years to get all the way through to full completion. So that's 1/4 of your runway gone right there.
Every year you delay is pushing that lower, and then there's whether the funding is available because you're in fairweather economic conditions or if crisis will happen concordantly with some other crisis (I.e. do you want to be stuck replacing air traffic control systems in a rush because some war has wiped out the floppy supply chain right as your air logistics is a critical issue?)
Every year you delay is pushing that lower, and then there's whether the funding is available because you're in fairweather economic conditions or if crisis will happen concordantly with some other crisis (I.e. do you want to be stuck replacing air traffic control systems in a rush because some war has wiped out the floppy supply chain right as your air logistics is a critical issue?)