Restrictions and Rules.I believe that the default version of the game is steadily moving away from a pure sandbox experience towards more of an adventure style game with RPG elements and some sandbox elements. But the days of the game being primarily a sandbox are past. As the game becomes more and more complete there will be rules and restrictions to facilitate balanced gameplay.
You can always change settings in order to make the game more like a sandbox such as turning off biome progression, turning on creative mode, adjusting block damage, etc.
This in a nutshell is where we are heading it seems, I agree. As long as the game is mod-able and customisable, I think you can carry your veterans with you. If that ended it may be a different kettle of fish.
The adventure game - non sandbox I suspect is to cater for newer crowd, along with console market. I mean I get it, money comes from new players and customers so the drive will be to follow the money.
What the game needs most is many more POI's, a broader spectrum of weapons, food, et al. At the end of the day repetition is the main reason folks will drift away I think.