Not OP, but I work on medical devices. One product at my last job had an expected service life of 20 years. FDA requires that the manufacturer maintain the ability to support and service a medical device for, IIRC, 5 years after market exit.
In the 10 years after release that I was on that project, we went through multiple OS upgrades from Windows NT to XP Embedded, to Windows Embedded Industry (replacement for XP Embedded) and a number of replacement x86 CPU boards had to be qualified as one manufacturer after another exited the market. Since the device is validated as a complete system, we often had to buy a year or two stockpile of existing product to give us time to start the Validation process for replacement hardware.
You usually have plenty of warning from a supplier that a product (Windows or a CPU board) is going EOL at a certain point, so you need to start validating whatever the next replacement will be well ahead of time.
If your project started 30 years ago, that means DOS, or Network or maybe one of the IBM behemoths?
Then the maintenance includes pacing OS updates and dependency changes?