In business terms, open-sourcing a piece of software they haven't touched for ten years means spending a number of hours on doing that for no obvious gain¹. Legal has to clear this for approval (risks? liability?), someone has to decide if opening up the code won't give away company IP, and someone has to actually find the source code (not a given at all), put a licence on it (going through legal again), upload it, and provide a minimum level of documentation.
Doing nothing on the other hand, costs nothing.
1: There is the positive marketing you gain from such a move, but someone has to quantify if doing this is worth it.
That's fair, but I still don't see why that somehow means that the OS vendor should therefore be expected to keep that code working in perpetuity. I have no love for Apple or MacOS, but I just don't see how they're ultimately the ones who should be blamed for this situation.
Doing nothing on the other hand, costs nothing.
1: There is the positive marketing you gain from such a move, but someone has to quantify if doing this is worth it.