This isn't surprising (or shouldn't be). Plane systems are very complex, have years long development cycles, and then are kept in service for many years, even several decades. Except for parts that are too good to obsolete - e.g. the LM358 and LM324 dual and quad op-amps - the production life cycles of ICs are much shorter. There are semiconductor companies "out there" whose entire business model is buying old equipment, masks, etc. from the likes of TI or On Semi or etc. for parts they are obsoleting and then producing those parts (usually for defense electronics companies). But even they have life cycle limits.
A previous employer was (still is) a defense electronics company. The development cycle for their products meant that they would produce the required fully operational prototypes, shut down that model's production for several years, do repairs as needed, and (years later) receive a production contract. The first step they had to do in that contract was to root through parts lists for obsolete parts and find replacements. Replacing an obsolete type of 0805 MLCC is easy; replacing an IC that is obsolete and has no direct replacement ... it's circuit redesign time.
Another joy of defense electronics is that the military buys what they want to buy over a few years, stops buying for a few years, and then wants to buy a bunch more. Result? Root through parts lists for obsolete parts and find replacements, if possible, again. I observed this at that previous employer.
Being a defense contractor can be a major PITA.
