AFAIK Iridium is part of some important airliner navigation systems and standards - while a niche, it can still be very lucrative business. and I would not be surprised if it was embedded like this into various other systems that are less cost sensitive.
Yep, it's one of only two satellite communications systems certified for both GMDSS/SOLAS and aviation operation and safety (ATC) use cases, and the only global one at that (the other one being Inmarsat/Viasat, which does not work near the poles due to being GEO based).
It took Iridium over a decade to get that certification; availability and political concerns are probably much larger in that segment than for e.g. home or passenger entertainment Internet use.
In the medium and long term, I can see the high-throughput LEO players eat Iridium's lunch for aviation, though; small antenna size (and the lower drag that goes with it) used to be their main advantage over Ku and Ka band offerings, but now most airlines want passenger connectivity anyway, and once you have that, the pressure to just get that certified for safety (with HF as backup, which you need anyway as far as I know) is going to be significant. The case for shipping is probably similar and even stronger.
yes, for example it's used on high altitude balloons.