CNC 1 from 2002 and 1 from 2007. Saw from 1997.
All have been maintained and still function well.
IMO the biggest potential issue with most machines is not mechanical but the electronics. Most mechanical parts, can generally be replaced with the same part or similar part from a different supplier. The exception would be parts designed for the particular machine for/from the manufacturer.
I have found more issue with the electronics. Some manufactures have proprietary electronic parts made for a specific machine. Thus there is a limited supply. Most are not going to have an electronic component for a machine that is 20 years old. Thus, you have to resort to the aftermarket and used parts. Most older machines are worth more as spare parts and scrap metal than as a complete operating machine.
An example would be the beam saw used in our shop. The saw is 22 years old and works well. The operator does a great job with preventative maintenance. Yet the PLC that controls the machine was manufactured by a company that dabbled in machine controls for a short time. The company is still in business, yet no longer makes controls for machines. As well as the saw runs today, if the PLC were to go bad, the saw is useless. The only PLC replacement would have to come from an identical saw that has been scrapped.