Does it work? Sure. You have to ask more questions. How much does it cost to keep it working? How much would it cost to upgrade? If we do nothing, along what sort of timeline can we expect it to stop working, or become cost prohibitive to maintain?
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?)
