Agree about the quality of life afterwards being the most important issue. I have seen too many people who have lingered "alive". I suspect that if I get knocked down then there is a dichotomy of outcomes: 1) unpleasant, painful, survivable, not time-critical, not life-changing 2) major trauma, life can be saved with a time-critical intervention. Prognosis post-event is for a decreased quality of life. Would probably die if left without unusually quick ER.

I can only see the watch being useful for #2: the exact situation where I think it would be better that I die.