The one thing I'll say that makes the whole "All-digital" AM different from AM Stereo is the availability of receivers. Lots of cars have HD radios. You can buy them in a lot of places. When AM Stereo came out and the 5 systems were competing for stations, there were very few receivers available because manufacturers didn't want to put money behind a system that didn't win. After AM had largely lost all music, you could find C-Quam receivers, but there really wasn't any reason to get one at that point.
I don't know what will happen if the FCC adopts this rule, but I'm okay with it. I'll mostly be interested to see how many stations flip to all-digital and in what markets that happens. It's hard to imagine that some big market popular AM station would do it, but there are plenty of good signals in big markets with low or no ratings that might.
Having been alive during the AM Stereo thing, I agree that the availability of HD radios is greater than it was with AM stereo radios when AM stereo was introduced. That doesn't mean HD radio will be a massive thing, though -- but it may be a factor in it lasting a bit longer.