and for a life saving device like this i know a subset of people will not want it a part of their phone.
...but this will definitely take a LOT of business from garmin because i believe that subset to be pretty small.
With that said, the iPhone (Pro?) is becoming somewhat of a swiss army knife for casual tramping: offline maps with GPS and a compass, an excellent set of cameras, an ereader, excellent battery life to back all that up _and_ it can now communicate with the rest of the world in non-life threatening -- but marginal -- situations. As an example, we leave intentions with people whenever we leave ("here is our route, if we're not back by midday on the third, contact SAR"); being able to message people and say "hey, I've rolled my ankle, we're safe but will be late because we're limping out, don't worry" is a pretty excellent feature.
We're probably a niche market, but several friends in that community are pretty excited about this feature. For 2-3 day trips, this makes the outdoors much more accessible.
Kind of like how Collision Detection could have been an excellent Apple Watch upsell feature but was brought to all iPhones anyway.
They recognize the power of things like fall and afib detection for pushing into the ever growing consumer Health market.