It will be a long time before you see private rooms or mods.
I don't know. An instance in an MMO (ex. a dungeon instance) is private. Only you and whomever you invite in with you will be in it. Just imagine the
entire world being instanced and it seems technically conceivable that each player could have the entire world to himself, if he wants. Imagine, then, being able to invite your friends to your instance, so that you can play co-op together without ever encountering another gamer who's not in your group. If Fallout 76 can manage to incorporate that capability, most of the angst should disappear.
It might even be the future of open world games. Eventually, they may all be online, even "single-player" games like The Elder Scrolls, but you'll have the choice of whether to play them solo (completely alone, just as if you're offline), in co-op with friends or with lots and lots of strangers. That could blur the distinction between single-player, co-op and massively multiplayer. They could all become the same except for how many other players you choose to allow into your experience.
The problem is that there are no NPCs except for some robots. So I'm curious to see how exactly they can tell a story without any npcs
Myst and many, many games since have told good stories without any NPCs, so it's possible, though telling a story through reading (as most of those have done) may not lend itself too well to an online game, so I'm still curious, like you.