Presumably, just aircraft IDs would be encrypted, and not data that is safety-critical for collision avoidance? Things like callsigns (which can be changed each flight and don't necessarily have to be the aircraft's registration) and weight category could also be added, unencrypted, so that ATC screens wouldn't go dark if there was some sort of key management failure.
There is also an argument for some sort of cryptographic signing of ADS-B messages in order to prevent spoofing.
Aircraft IDs are used for type lookups and uniqueness. Callsigns are already in the signal and must be the registration for anything other than commercial or military flights, as they're registered and there's a process for allocation. Weight category and other data being added would require a whole new revision of the standard and will not reach most systems for decades.
Is all of this really necessary when billionaires have other means at their disposal to avoid scrutiny? Why add risk and complexity to a critical system just to make their lives easier?
Spoofing is not really a useful attack vector, for various reasons. In any case it's also not something that can easily be retrofitted for the same reasons, in that it takes decades to update these systems.
There is also an argument for some sort of cryptographic signing of ADS-B messages in order to prevent spoofing.