Kill AM

[A Black Falcon]

This is technically feasible, but not even remotely practical. I can't see many people spending $40 on an extra device just to listen to the radio. People might listen to radio because it's free and always there, but how many do you really think are going to not only pay for the privilege, but also have another device hanging out of the deck? I'd guess maybe 1 in 20.

Digital TV transitioned easily not because of converter boxes, but because cable and satellite had gone digital years before the mandated switch, and most people have one or the other. Many of them were totally unaffected by the change.

Also, a fairly large number of mid-range car stereos have no means of external input.

That's a very important point. Neither I nor my parents, etc. have any HTDVs or digital TVs in general, but we do have the basic cable service, so we haven't needed to get any new hardware to keep watching TV. With radio there is no such easy way, everyone would be required to buy something.

Also, as you say, yes, many radios do not have any external input. It's not just many kinds of car radios, but no clock radio has internal input either, and those are extremely common and popular... no, sorry DJ, we can't get rid of radio anytime soon, I think.