I don’t know about Elite Dangerous, but in the case of No man’s sky it’s just simply a plain terrible game in every regard. I’m not sure it’s fair to let it tar the whole of the space sim genre.
I don’t think that the point of a game like Elite dangerous is really a strong cast of characters and an engaging story to play through. The point is to create those sort of memorable events yourself, how good the game is at offering that I don’t really know. I do remember Freelancer, at the time, felt like a huge well realized universe you could do whatever you wanted in. Still couldn’t say I remember any NPCs in it but it was a great game for “roleplaying” a star traveling explorer, trader, mercenary or whatever you wanted to be. I have not played the X games myself but I believe they are also very good large simulated universes.
As for if it’s possible to emulate a proper universe, I think it might be, the old Elite games managed some how. The problem is that we’re all a bit hung up on graphics and that’s something that really bogs what a game can become down. A good example that showcases this is Dwarf Fortress, the level of simulation that goes on in that game is absolutely staggering and while it’s “just” one planet I’m pretty sure if someone wanted to do it it could be applied to a universe.
But yeah probably wouldn’t be possible to create that and also have beautiful 3D graphics representing everything being simulated.