Garmin and Iridium will still sell well with the people who are cruising in relatively low cost sailboats. These people can't afford $5000/mo just for internet access.
Garmin doesn’t operate a satellite network. They produce devices and on sell subscriptions to other satellite services. This won’t really impact them at all. They could pivot to use another network if it becomes more feasible.
RIP Garmin, Iridium, etc...
As to why it is expensive? Well, they did their homework and found out is a lucrative market (one doesn't need a lot of hindsight for that, though).