HD Radio would never kill cellphones, but it will kill itself and has brought traditional radio down with it, especially AM radio. But again traditional radio is over 100 years old and has to compete with 5G technology. Just wait and see where radio will be in the next decade, my guess radio will be mostly streaming. Streaming is the true digital radio not HD Radio or should we say Hybrid Digital which is like fake bacon.
I don't think HD Radio has had any bearing on the decline of radio. It's more a matter of the radio marketplace, and the associated lack of investment. Audio programming in general has lost some audience to richer media. Radio, in particular now has to share the marketplace for audio programming with streaming and podcasts. Radio is just a medium for audio programming.
As for the promise of 5G and the smart devices attached to it, I'm still not getting rid of my radios. They have their place.
The thing with the smart devices are that they are subscription based, one way or the other. You don't get service unless you subscribe. Once you buy a radio, it will work till it falls apart, becomes obsolescent, or if your local stations die off.
Smart devices have a relatively short lifespan compared to radios. They become obsolescent because of software bloat or abandoned hardware. In comparison, I have a late 1960s era Sears Silvertone tabletop radio that still works fine for analog broadcast radio. It won't be obsolescent until radio goes all-digital. I'd like to think that the HD radios I've bought over the last few years will have a useful life measured in double digits.
I'm not trying to say I have a problem with innovation. It's pushing us forward and I use smart devices all the time. Radio still has a place in my life as a way to consume audio programming without all the fuss of managing credentials, subscriptions, or the upgrade treadmill for the smart devices.