"Unless there's additional information I'm unaware of, it seems as if the design and/or manufacture of the radar altimeter is not fit for purpose."Whether radar altimeters in their present form are fit for purpose or not I cannot answer as I have insufficient information to do so. However, I must point out that:
1. ALL radiocommunications have limitations in that they can suffer interference from other services even when those other services are operating correctly and according to license (i.e.: being in their correct allocated frequency bands and operating at their correct power levels, etc.).
2. The role of Spectrum Management falls to the FCC. It operates under the auspices of the ITU (International Telecommunications Union) to manage the spectrum through its international treaties and its job is to ensure that interference between services is kept to a minimum. In essence, the aim is to maximize spectrum usage whilst keeping interference to trivial levels (it should be noted however there's no such thing as 'no interference').
(International treaties are necessary as electromagnetic radiation doesn't honor a country's borders.)
3. The way this is achieved is complex but in essence transmitter frequencies, power levels and transmitter locations are optimized to provide optimal protection ratios to each service so they can operate efficiently.
4. If services are mobile, as with radar altimeters, one of the parameters used in optimizing protection from interference is lost. Signals from radar altimeters can interfere with other services and vice versa as they can get too close to one another (interference increases significantly when powerful transmitters are close to sensitive receivers).
5. How tightly services can be packed together depends on many factors but two principal matters stand out: the laws of physics and the state of the art of electronics design.
The major culprit in causing interference is a well known form of distortion whereby several radio signals can interfere with one another to produce multiple other freqenies that are harmonically related to the original frequencies. This interfere is called intermodulation distortion and it's one of the perennial scourges of radiocommunications: https://en.m.wikipedia.org/wiki/Intermodulation.
For example, a transmitter on f1 (service 1) and another say on f2 (servive 2) can produce other unwanted frequencies, f3, f4, f5, etc. that can interfere severely with unrelated services. Under some circumstances interference to the unrelated service can be so severe that it can be rendered useless (as mentioned, this can be very problematic when services are mobile as it's very difficult to take every contingency or location into account).
6. These matters become much more critical when the stakes are high - with emergency services, radar altimeters, etc. where lives can be at stake if things go wrong.
7. Critical communications services have always demanded very high protection ratios and have traditionally been given them by Spectrum Management agencies - the FCC, et al. The current argument is that the ubiquity of 5G threatens the high protection ratio previously afforded radar altimeters.
8. If I were in the aircraft industry and I was responsible for the reliability of safety devices such as radio altimeters then I'd want to be absolutely certain that any new radiocommunication service is not going to interfere with any of my critical services. Moreover, I would be overly concerned with the potential for mobile phone services to interfere with these safety services specifically because phone services are a never-ending moving target when it comes to location, density of phone traffic, etc., all of which have potential to cause interference.
9. Whether 5G has real potential to interfere with radar altimeters or any other aeronautical communications service can only be determined by proper independent engineering studies.
The fact that we have this dispute at this late juncture shows that something has seriously gone wrong in the planning phase.
(From my observations, it seems the mobile phone industry has had too much political power, especially so when it comes to buying spectrum - and it has lots of money to demand what it wants (and to date it's gotten the spectrum it wants). Whilst government eyes the millions to be made from the sale of spectrum, the trouble is that it's also the regulator of the spectrum hence it has a conflict of interest.
Fundamentally, spectrum auctions run by governments have the potential to compromise good spectrum management. Whether it's happened in this instance I cannot say for certain.
The only other point I'd stress is that the electromagnetic spectrum is a scarce nonrenewable environmental resource and should be respected and treated as such (like any other environmental issue).