But I feel that encrypting the protocol goes against the spirit with
which I, and many other early adopters made risk investments.

As a matter of fact a FLARM customer might very well receive this communication soon:

"Dear Flarm customer, as already announced four years ago the new firmware release 6.0 prevents DSX devices from exchanging position, speed and altitude data with our products operating this release. In case of a mid air collision we do not take responsibility for the DSX pilot as he/she chose to buy an incompatible device. As for that one of our customers, who will happen to be involved in accident we sincerely apologize for the inconvenience. Thanks for choosing FLARM."