I definitely see and agree with a lot of your points. But I'd like to respond to the both of you first in this one way.
I think that what I'm claiming is that both SAO and Index are having reduced sales because in this hypothetical there would be extra digital sales which represent DL novels of SAO and Index being bought which we can't see because they aren't being tracked by Oricon. Maybe that isn't the case, but that was what I was trying to imply. I'm not just saying that Oricon only does this to Index. That would just be a desperate, silly theory from a fan attempting to escape from reality.
I just wish we could learn more about how Oricon tabulates sales. That'd help answer our questions.
Either way, I think Index III could hugely boost sales. Regardless of whether people think it will be overlooked or not. If it looks amazing enough, I think it could draw people into the series. Not to mention, if it ever manages to surpass that animation threshold of this is so good that you should slog through two seasons of anime to watch this because it is done so well, then it gets easier to try to convince people to watch the shows, since it'll be easier to prove your point that amazing stuff from the series exists. You only need to show a short scene or two out of context and you're good...
Of course, I agree that Durarara's situation is very different from Index's situation. But I also do believe that if Index ever got a season like FSN has been animated, then it would do much better. I don't think there's anyone who used to watch this series that will refuse to watch season 3. Everyone knows that season 3 is when things get really good, and I think just that could easily guarantee that old fans would be willing to shell out money on it even if they aren't fans of the series anymore because of dying hype.
But I think you worded it quite well, it's inconvenience, and a lack of desire to try something new and different that often makes people too uncomfortable or unmotivated to go out of their way to try reading something like Index even upon recommendation...