I think the FCC's own refusal to ensure interference from part 15 devices is what is really killing AM, not to mention the lack of quality receivers. AM Stereo alone cannot save AM.
It has two fundamental problems common to terrestrial radio:
1)-a lack of compelling LOCAL CONTENT. AM,FM, HD...doesn't matter, if nothing attracts people to WANT to tune in, they won't. It isn't 1979. We now have an abundance of media on tap thanks to mobile Internet. Corporate radio is stagnant, dull, unoriginal, and irrelevant to most people today.
2)-Interference. AM to more extent, but even the FM dial is full of interference. Even if there IS great content, if no one can hear it, they will just go back to their Internet stream which is usually clean, free of static, buzzing, crackling, etc. That is what today and tomorrow's radio listener expects. In many markets, thanks to the great translator SCAM, brought to us by Radio Assist Ministry and Edgewater Broadcasting (or as I call them, TURDWATER broadcasting) we now have translators filling every possible unused FM channel. Most of these are move-ins and very few have any content worth listening to. Nevermind that they skirt the law re-transmitting HD2/HD3/HD4 of other full service FMs in the SAME city grade contours, the corporate casters bend the rules, and the FCC doesn't care. The FM band is polluted as a result.
In band on carrier is the WORST idea, a technical abortion, and everyone knows it. HD on AM makes DX impossible, and even daytime reception of analog stations unbearable. Sadly, the wider your IF bandwidth is, and IBOC station actually sounds WORSE on a GOOD wideband audio capable AM set. the constant whine of the IBOC sidebands destroys otherwise acceptable audio.
On FM, the IBOC sidebands kill adjacent channel listening on many radios.
IBOC is a joke, it should be turned off. It was never a great idea and never should have seen the runway. But as we are in a corporate owned country, the FCC is at the whim of whoever has the biggest checkbook, technical standards are molded as the corporate paymasters see fit, tossing aside all laws of physics and RF propagation.