You have drawings that show how it works and a supply of spare parts. When it stops working people can figure out why using the drawings and then fix or replace the broken or worn out components.
No I guess thats not so easy. Every new part in a design for a nuclear power station has to be certified. So just use another almost same chip is not so easy, because you have to make o lot of tests, so the new part is allowed. Thats why a lot of things for nuclear power station are so extreme expensive.
The components we are installing have chips in them, but we don’t worry about what chip because the manufacturer of the product keeps it working the same way. The modicon quantum Plc uses the pentium 166 chip and was released in 1994 and I can still buy it today from Schneider although they encourage new installations to use a newer product. I am sure I will still be encountering old quantum Plc in the field when I retire, the platform will have a 50 year lifespan.
Even when a line of controllers is end of life the manufacturers provide an upgrade path to their newer product line.