I think if No Man's Sky attempted to create a Journey-esque experience it wouldn't be very effective. Journey's second player worked because the environments were relatively small so it was easy to find another player when traversing through the level. No Man's Sky's planets can take literally months or perhaps even years to walk across. The planet sizes are comparable to actual planets. It would be extremely unlikely that you'd just stumble across another player.
That said, I think the shared database aspect of the game is potentially exciting. You could be the first person to discover a planet or a new species, a discovery which is then shared with every other player.
I should clarify that I'm still pro structured, linear games. Indeed, my top ten list mostly consists of games which are highly structured, complete, linear experiences. Regarding story, the problem is that most games, as structured experiences that tell a story, well... they suck at it! Books, films, and other media are typically much better at telling a linear narrative because they don't have to have trade offs between the gameplay and the story; they can be all about story. If we consider a number of linear, story-driven games that have come out in recent years - Uncharted, Tomb Raider, Halo, Call of Duty, etc - these games have passable storylines at best. But the story usually does very little to address the plot element that you're playing as a serial killer and the consequences of that (the Last of Us is a notable exception to this rule). Most games are about combat, or some other non-story related gameplay mechanic such as puzzles or resource gathering, so they have this problem of putting the gameplay and story in seperate boxes. It also limits game stories to a certain type of story. There are exceptions, such as Life is Strange, but these games are few-and-far between (outside of the indie games). So for most games, I think it's worth questioning whether it's even worth playing games to "progress through the main story".
Non-linear, exploration games have the advantage of not having to tell a story. You create your own story through your actions. Indeed, I would argue that if you're playing these games with an predefined objective in mind (story or other game-related goal), you're playing them wrong. I talked about this more in the Bethesda games topic, but to summarise, the appeal of these games is the freedom they provide not to have an objective imposed on you by the developer. You have to let go of the goal-oriented, efficiency based approach that we often take towards games (and life in general) and live in the present a bit more to appreciate what the game world has to offer, I think. I don't play open world games with the intention of finishing them. It wouldn't matter if I didn't finish them (whereas by starting a story based game the intention is normally to finish it). I enter these worlds for an hour or two at a time, often as a means to relax.
The attraction for me is the geography of the game world, of traversing through the world without a specific objective, but coming across new locales and events within that world, but having the choice as to whether I participate. For example in Fallout 4 I have completed very few quests, but spent over 40 hours just exploring the world and I'm still stumbling across new things. But you could easily have good time just spending 5 or 10 hours exploring if the goal isn't to complete anything. Sure, there is certainly some repetition as you wander around, but the joy of discovering something new (without necessarily intending to) things makes up for that.
Regarding No Man's Sky, I do think this is legitimate concern that there may not be enough in the world to keep the game interesting for long. It's certainly ticking the boxes that I like though - large expansive worlds, the ability to play how I want to (no requirement for combat), survival aspects, and options for multiple approaches to progress.
MMORPGs are whole other beast. When I played World of Warcraft (for two months) it became clear very quickly that these games are designed to prolong the experience in order to keep you paying that monthly subscription fee, rather than because the extended experience is geniunely enjoyable. And if you don't play the game enough during the month, there's the feeling that you're wasting your money playing the subscription fee. Whereas with single player games, the developer already has your money, so there's no need to drag out the experience (DLC is another matter). So, I typically avoid MMOs (and free to play games) like the plague.