While doing a full readthrough of the 1980 Expert Set, I came upon this little paragraph in the Introduction chapter. (Which probably a huge number of people never read.)
Most important, the characters in the wilderness campaign do not exist in a vacuum. The DM should have events going on elsewhere that may affect (or be affected by) the actions of the players. There may be any number of “plots” going on at once, and the DM should try to involve each player in some chain of events. These should develop logically from the actions of those involved. It is important not to force the action to a pre-determined conclusion. The plot lines can always be adjusted for the actions of the players.
Emphasis not mine.
This isn’t any world shattering wisdom here, but you could easily call it the most condensed explanation of the current consensus regarding sandbox campaigns, especially among OSR grognards. Even with all the dozens of articles written on the topic and probably thousands of forum posts, it doesn’t really seem like we’ve discovered anything truly new about the understanding of how these types of games work. It’s all aready there, 35 years ago. This quote even predates the release of X1: The Isle of Dread by a few months, so he was talking about something most people actually hadn’t heard of before.
Strange that I’ve never seen this quote in any discussions about sandbox campaigns before.