> Also, if all it takes to mess with altimeters is a rogue 5G cell, the right approach is to harden the altimeters, instead of hoping no 5G cell ever will be misconfigured or pushed wrong configuation parameters by a enemy country or ransomware hacker group.
Wow, that is a lot of misconception about the threat model and real issue at hand packed into a single sentence.
First of all, great, let's say you're right. How do you harden the existing altimeters out there, equipped on thousands of aircraft? These are sensitive, tightly controlled and life-critical instruments. Radar altimeters have been in use for decades, almost a century now, and so the FAA is for the most part talking about already installed altimeters which were designed far before 5G was ever a thing. Likely most of their designs will not readily accommodate hardening against new kinds of RF interference. If you managed to patch all the different kinds out there and get fixes pushed out, that'd be a great effort in labor, logistics, and certification. Of course, one could simply replace the old radar altimeters with new ones, at a much greater cost too.
Separately from the difficulty of actual hardening, there's the question to be asked of, why ask old technology to adapt to new one when we can adapt the new technology to suit the old? Especially because, again, these are safety critical instruments that you don't wanna mess with, whereas 5G towers are much better suited to new experiments or fixes, or just being designed correctly and safely from the get go when the technology hasn't even launched yet.
And then, why do you think this is a security issue, as in an attacker aiming to disrupt air travel? You must be aware that most all aviation radio comms are in the clear and extremely easy to jam and spoof, including critical instrument systems like ILS. That is not the threat model! The FAA is worried about interference from ordinary people trying to use their cellphones, rather than a specialized attacker targeting radar altimeters. Of course, because these are two very different situations!