Unless there is a certain number of a specific aircraft in the world-wide fleet of that aircraft it is impossible to maintain them to an acceptable airworthiness standard. The reason is that the overhaul facilities of the engines and other components cannot economically maintain the parts, manuals, and test benches to overhaul the components. Even if you have a large warehouse of overhauled parts many of those parts will have time -limited components themselves which require retesting and recalibration after a certain number of years even though they have never been used. They cannot be recalibrated or checked in many instances because the test benches and manuals have been discarded. With the ejection seat that was mentioned I would hazard a guess Martin-Baker no longer maintains that model seat. They might, or might not, retrofit a seat for you but my sense is that it would cost a very large amount of money and the operator would incur a continuous maintenance program just for the seat. And we have not even begun to discuss if the airframe and engine manufacturer provide service such as updated airframe and engine manuals.
There is no other reason, except cheapness on the part of the operator, to keep using these antiques. Fly them at an airshow on a sunny day under an experimental certificate if you want to. Anything else, no.