Isn't this what MUDs are? I tried a few in the early days of the internet and even back then they were like much bigger and more dynamic versions of text adventures of the 80s. For me I bounced off the idea that I had to role-play with other humans - I thought it was far more interesting to chat with other humans about real-world topics - but if you are looking for a large, text-based role-play experience then it's probably worth trying out a few. There might even be some that can be soloed these days, there are so many.

I think the challenge of trying to make an "endless" game using an LLM is the same challenge that all procgen games face - they are boring for people who are seeking a well-paced narrative. There are players who enjoy the mechanics of looting/crafting/trading/etc who will gladly play games where the story is incidental or emergent, but if you're specifically looking for something with a bit more narrative depth, I'm not sure procgen will ever work. Even if there is a system that tries to project coherent storylines onto the generated world, you still need the player to do things that fit into a storyline (and not break the world in such a way that it undermines the storyline!), otherwise the pacing will be off. But if the system forces the player into a storyline, then it breaks the illusion that the world was ever truly open. So you can't have it both ways - either there is a narrative arc that the player submits to, or the player is building their own narrative inside a sandbox.

AAA games try to have it both ways, of course, but it's always pretty clear when you are walking through procgen locations and leafing through stacks of irrelevant lore vs when you are playing a bespoke storyline mission that meaningfully progresses the state of the world.