Radio receivers are not being purchased because the number of listeners have declined.
That is not as dramatically correct as you seem to think. In 1990, around the peak in both percentage of listeners and length of listening, 94% of adults listened each week. Today, that figure is around 89%. That is a loss of approximately 5% in radio's total cume over the last 30 years. f
The issue with radio sets is that people like multi-purpose devices such as phones that give them access to music, news, social contact, photos, videos and a whole bunch of other stuff. A single-purpose device is not attractive.
As many have posted, the AM band is in decline due to a more noisy environment (the downfall probably helped more by the prevalent right-wing programming crap). The FM band, apart from many PBS stations, is becoming more boring.
Actually, the conservative talk sector became the rebirth of AM radio starting over 30 years ago. AM would be almost totally dead today were it not for Rush and his camp followers who have kept AM alive for three decades. Of course, that was made possible by the elimination of the Fairness Doctrine that allowed polarized station formats.
IBOC HD Radio works well on FM, but has been a failure on AM. DRM is a better technical, and less expensive, alternative to HD Radio.
And DRM has only been accepted for broadcasting on AM where the government has mandated it and built all the stations, specifically in India. Conceptually, it was developed to enhance Short Wave, but those bands are pretty much dead now.
Introducing DRM, with its neat features, together with interesting programs, would provide a demand for the new radios.
DRM requires old stations to stop broadcasting and to be replaced. In India, the government simply added additional facilities, many in the 100 to 1000 kw power range, as they alone control the AM band there. There is no way to convert to DRM in the US as few would buy new home radios and it would take over two decades to bring it in to all automobiles.
The main competitors to radio broadcasting now are the streaming services (via cell phones). DRM can provide an easier noise free reception, but the programming will still be a deciding factor.
DRM can't happen here, as no station is going to run with no listeners for years while the auto industry decides to add DRM to car radios. In the meantime, few will buy stand-alone home radios to get stations they don't like anyway.