And I guess you are writing just to the young users. There are a lot of old people living alone (in the part of the world where such watches are affordable). Falling, even in their own house, then remaining immobile and unable to call for help is a common cause of huge problems (1) (2).
The specialized devices already exist too, but a lot of people don't carry them, until they already suffer such event and also have luck to survive. Such people (who would otherwise not survive) get potentially (if the system do engage and what follows can be influenced) a huge benefit in a watch that has such functionality.
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1) https://www.medicalalertadvice.com/fall-detection/ also:
"With the monthly costs nearly twice that of a traditional medical alert monitoring system, the cost alone may be the deciding factor. Is the extra costs worth it? Only you can make that decision. Keep in mind, these emergency alert systems cannot detect 100% of all falls."
2) Also see the exponential increase of chances to die with age:
https://en.wikipedia.org/wiki/Gompertz%E2%80%93Makeham_law_o...