I've been waiting for this feature since it was first rumored a few years ago. It's perfect for my backpacking trips!
The iPhone is a terrible alternative to a dedicated PLB for multiple reasons. I would never trust an iPhone for emergency communication in the backcountry for several reasons:
First and foremost, battery life. My SPOT beacon lasts for weeks, if not longer, when it's in tracking mode all day on the trail. For those who aren't familiar, SPOT has a feature that records your route in realtime and allows others to view your location via a website. Friends and family enjoy seeing where I am on the trail and it's fun to have a record of my route at the end of the trip, especially since a lot of my trips are off-trail. When I record a GPS track with my iPhone, the battery doesn't even last a day.
Second, the iPhone battery also fails in cold weather and higher altitudes. The replaceable lithium ion AAA batteries in the SPOT don't fail in such conditions.
Third, when using a dedicated PLB like SPOT, one press of the Emergency button sends my location and dispatches Search and Rescue. Will the iPhone automatically do this? Or do I text my friends and try to explain where I am and have them call SAR? TBD.
Fourth, my annual SPOT fee includes evacuation insurance. If they need to send a helicopter, I don't pay a dime. Will Apple offer a similar service? TBD. If they do, it will surely cost extra.
Fifth, PLBs are extremely durable. iPhone, not so much. I trust my SPOT. It's never failed me in over 10 years of use. I would never trust my iPhone in a critical backcountry emergency situation. It's certainly not a bad backup option, but a dedicated PLB should always be the first option.
Sixth, my SPOT service works all over the globe. As does the SPOT evac insurance. What will Apple offer and how much will the service side of the equation cost? Will it be global? TBD.
Personally I think this feature is a bit of a gimmick. If anything it will give certain people a false sense of security, but mostly I imagine throngs of annoying Bay Area techies coming up here to the Sierra trying to send selfies to their friends from remote locations.