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Today i was on some random dudes multiplayer server. I was the first to log in and this guy literally build a castle made of gold and red bricks with all these statues and really cool stuff.
I checked it out and went on my way to add to his little kingdom.
Twenty minutes later two more people logged in.
They see the structure and immediately start breaking it apart for the gold bricks, statues, banner and all the other stuff and log off...
While a sandbox experience as a mmo sounds like an amazing thing on paper there is one fatal flaw.
You can't have a complete free-form experince because people simply dont know how to act, they need to be directed like children, be punished for bad and rewarded for good.
You can't have a sandbox mmo without hard coded mechanics to curb peoples asshatery. And once you add in those mechanics a game is no longer a free form sandbox.