Oh, come on. I'm not saying that nobody should know which random ID corresponds to which plane. That would be ridiculous. Just keep that index on an FAA (or whoever) computer system networked to the control towers at various airports, or wherever it needs to go.
All I'm saying is that there's no technical reason why that info has to be broadcast over the air, unencrypted, for anyone with a radio to hear. If we wanted to fix that, we could. This just seems like an obviously correct statement to me. I'm not saying that we should or shouldn't want to. Maybe we shouldn't. But we could. It is within our power.
I guess people just seem to find the "privacy for me but not for thee" angle too delicious to give up and feel the need to justify it to themselves? That's the only reason I can think that someone would say something like "the data is impossible to protect" with a straight face. Politics brainworms in action.