Thanks for your comprehensive answer Marcel!
I guess an app like that is all about gathering enough data to draw behavioural patterns and conclusions, which can be used as guidelines. You probably already know the app, which on the basis of your health data can predict e.g. a heart attack. So the same is absolutely possible for an app targeting runners and the like.
Very interesting reasons regarding the decreasing number of members! I have for instance ever been a member of a club due to reasons number 1 and 3, but as the same time I find it thought-provoking that the western world seems to be so self-centred and deselect social activities. I have no data to underpin this fear, but I fear that in a few years the only social activity is at work and in the supermarket. The rest of the time we live in a self-centred bubble behind a screen.
But again, this is only a layman´s fear.