That's where the manufacturers have a role in making HD Radio succeed. Every radio sold for the US market should be HD capable. By churning out analog-only radios, they are just compounding the problem.
You are missing the point: the average vehicle (where 50% of radio listening takes place) is nearly 11 years old. It would take another decade for just 50% of the non-HD car radios to be replaced with ones that are digital. In the meantime, a successful AM station that goes digital loses about 90% of their in-car listening potential.
And home and work radios are not being replaced or purchased. Old radios last "forever" and more and more people are moving to smart devices and like Alexa and their smartphone for audio. AM stations will simply lose everything there.
The FCC has approved HD Radio for use. All they can do now is refine the regulations to better support the private sector. Now it's up to Xperi, broadcasters, manufacturers, and retailers to do their part in making HD Radio succeed. Unlike other countries with governments that largely control broadcast, the US private sector will write the story of HD Radio's success or failure.
For AM, it is already a failure. Cross it off. There are less than 100 stations on AM... out of nearly 5,000... running HD. The majority that had HD at the beginning have equipment that long ago reached end of life and no AM will spend for new gear today.
And, again, most AMs do not cover their market today. As stated before, in the top 100 markets, there are less than 175 total stations that cover 80% of the metro survey area day and night. The rest are hopelessly inferior and not competitive today.