Most of the misgivings in this thread seem to be from not having full information.
The way it can be an MMO and not an MMO is that you have the option to host your own private server that still contains a persistent universe.
Chris Roberts: “We hate DRM, so the intention is to be DRM free. For the persistent universe you’re going to need to log with your account as we need to be the keepers of location, equipment and status for the obvious reasons. And if you want to have friends drop into your single player experience you will also have to be connected to our servers (as this is how we match make you with your friends) But you can play the single player game offline with no DRM and you’ll also be able to run your own server without any need to connect to our servers. Hope that helps!”
You want the sandbox in single player or co-op with just friends? Host a private server.