All the threats mentioned there - jamming, spoofing, impersonation, etc. - can already happen with the unencrypted radio calls it's supplementing. With a hundred dollars or so worth of radio, you can hop on your local airport's ATC frequency and hold the mic down, or pose as the controller.

I suppose the argument is, when there's a person in the loop it's easier to notice funny business. If someone hops on the ATC frequency, the odds are someone will notice more quickly.

When it's mediated by a computer, you tend to trust it as a magic box of truth- and injecting subtle errors is going to be easier with ADS-B than on a voice channel, isn't it?