Right, requiring human intervention to share a run (other than maybe with eg a specific small circle of mutual friends) seems like it solves all those problems, other than perhaps being annoyed that you forgot to manually share a run.

But at least that's a failure you can fix once you notice, as opposed to making something public that shouldn't have been. Letting people opt in to automatically sharing runs to the public just seems like something designed to get people to share stuff without thinking about it.