Stereo itself isn't going to increase the fidelity. The main reason a LOT of AM-Stereo radios and stations..the vintage ones at least...sounded better was the fact the AM stereo radios typically had a super wide IF. FCC specifications allow an AM station to have a REALLY wide bandwidth; the emission standards don't specify any attenuation until 10khz out from the carrier; this means the first 10khz of the signal requires no special handling. From there, there are sideband requirements...but that doesn't mean a station couldn't broadcast a 30khz wide signal...they just have to make sure sideband emissions are so many dB below the carrier after 10khz. (Remember, AM has sidebands on each side of the carrier, so a 10khz wide audio signal would use 20khz of RF spectrum.)
Some AM stations use the entire 10khz of audio...some actually don't. HDRadio is actually making it difficult due to the fact it widens a stations channel and some stations are actually having to narrow bandwidth to avoid interference.
But the biggest factor is no one makes an AM radio with a 20khz IF...and a lot of "wide" tuners I've heard aren't MUCH wider. They're a far cry from hearing something at it's full width.
https://dewdude.ath.cx/wkcw.mp3 is a recording I made of the local 22kW (daytime) classic hits station on 1420. I did this using one of those super cheap RTL-SDR dongles with a frequency upconverter a couple years ago. Yes, it's noisy because the RTL platform was NOT designed to be picking up AM radio...let alone anything other than digital UHF tv; this was also just a lousy piece of longwire for an antenna. But since all demodulation is done in software....I could easily set the bandwidth to the full 20khz.
HDRadio causes some serious incompatibilities with CQuam...mostly because CQuam uses phase modulation in addition to amplitude modulation. The phase modulation can apparently screw up the HDRadio sidebands...back when HDRadio was forbidden at night...most of the systems transmitted a CQuam compatible signal as the HDradio chipsets are *somewhat* CQuam compatible.
AM is on it's way out. It's a sad fact, but it's true. The revitalization efforts do nothing to solve the problem long-term and, instead, just start letting everyone pump out more power. Systems like HDRadio make it difficult to listen to analog systems...and the expense of and "failure" of HDradio means no one wants to support it. But I'll also point out the only people using HDradio these days are "the big broadcasters"...most of which are billions in debt and either bankrupt, restructuring, or dragging the process out. Remember, as much as HDradio raises the prices on a receiver; stations have to pay a yearly license fee to iBiquity for the privilege of using it. ARound here...pretty much all of the small stations that ran it for a while stopped...it was too expensive with no payback. No one wants to invest in a new radio with smartphones.
What we really need is a new radio system built from the ground up to be digital and using all open standards. It has worked fantastically in other countries. I have no clue why we're holding on to this outdated and severly limited idea.