I guess Frank has been talking to receiver manufacturers to see what interest there might be in a SSB decoder mode. Hypothetically this mode would narrow the filters ahead of the L-R demod. It seems likely that an improvement in adjacent channel IBOC induced noise would be another benefit. This wouldn't help the legacy receivers. But... most people keep their cars 5-10 years and in that time we could see advanced decoders take over a majority of the car radios.
I see a lot of concern about those early non-PLL receivers. I wonder how many of those there really are out there. The time between the wide consumer acceptance of FM Stereo and the end of non-PLL receiver production was very brief.
I'm old enough to remember the time right after the stereo wars where resolved by the FCC. Most of the stereo "decoders" were separate boxes that plugged into a special composite jack on the back of Hi-Fi receivers (the jacks were put there by manufacturers that didn't know which side the FCC was going to come down on). Most cars had AM radios and if you worked at an FM station like I did then, you had a Motorola FM receiver that went in the antenna line of your car radio to re-modulate the FM station onto the AM band. Certainly nothing stereo in cars then. I use to amaze my friends by driving through a tunnel and not have the radio fade out.