I can see MP being extremely fun in a few ways.
For instance, 100 players share Earth and Luna in a dedicated server environment. I think this could be fun, even without warp, so long as players had something to do, like dock to build stations and construct colonies, mine, and enrich the world. Or fight it out...
Perhaps I'm alone on this, but if it takes three real-world days to get to Luna, then so be it. Automations can handle insertion burns. There's a long-game element that's yet to be explored. Design and build your launch platforms, movers, and constructors. Get all of your assets built, launched, and into position, and your team creates a colony and infrastructure.
Imagine you and 50 friends all work to plan, build, and launch programs to colonize Luna. It shouldn't take a snap of a finger to do this. You'd need to engineer modules, schedule launches, and perform assembly. Things can and should go wrong and there's a penalty for pilot error.
Or alternatively, we get a weapons mode and you have a planet-scale 50 vs 50 combined arms conflict with player-built war machines
The game world is a big place. It may still feel empty if players aren't kept in relative proximity. Suppose you could help that in a persistent world with networked shards. In that way, a game world instance might be able to support a ton of players, but I say that more as a dreamcatcher.
It is an old discussion, and I figure devs probably put some good thought into this already, so I'm excited to see what they come up with.