Yeah, I think that automated choices about this are unlikely to work well. We chose the syntax for CREATE STATISTICS with an eye to allowing users to declaratively tell us to collect stats about specific joins, and I still think that's a more promising approach. But nobody's yet worked out any details.
Per other response, no, I didn't envision stats on all possible joins or even all possible foreign keys, just the ones we declare as interesting, and even then only for the attributes that we say are interesting on the far side of the join.
