It's only obsolete if it doesn't have the features you need. Which, in this case, include radiation resistance, which is much harder, if not impossible, to ach

It's only obsolete if it doesn't have the features you need. Which, in this case, include radiation resistance, which is much harder, if not impossible, to achieve with modern chips with very tiny transistors. Large transistors can much more easily shrug it off. The old chip is not just "good enough". It's the only thing that works.