MW listening in the US is 30% car, 30% home and 30% at work.
There are probably enough HD radios in new cars to provide an audience. The problem is that those newer radios also have bluetooth, streaming capability, etc., which are probably more popular than a digital AM station -- or even an FM station, for that matter...
The new rule won't change anything for the vast majority of stations, which, right now, are probably hoping they can stay on the air due to economic conditions, much less worry about buying new digital broadcast equipment. Maybe in 20 years we'll see a handful of digital AM stations nationwide, maybe a couple hundred analog AM stations, and the rest will be static.