I think the "game loop" is why I have stuck with 7 days 2100 hours later. "Here is a sandbox, but something is going to come F it up in 7 days, so get ready!". I don't think there is another game that gave me more of a reason to stick around for day 7, 14, 959, etc. Trying new things, trying those things again when a new update comes out. I wonder when the game is "gold", will I eventually become bored with it to the point I put it aside for a considerable amount of time?
That being said, other sand box games don't grab me much in the same way without that "pressure" the recurring 7 days is to me. I've played hundreds of hours of Ark, perhaps more of Minecraft, but I never felt the same urgency and implied will to keep going, try new things. It's a fantastic game mechanic, and I haven't been able to tear myself away very long for over a year now.
Here is hoping the final vision of the game stands as a fitting monument to the game play at it's core.