It’s possible, though arduous, to maintain it all solo. The game fragments who sees what so even at peak you generally only saw things built by a small subsection of the playerbase, so no one ended up presented with just everything already done and nothing to do. Steam is only showing daily peaks of about 1000, but that’s still more than enough for a healthy mix of established and unbuilt infrastructure, I’d expect.

edit: And actually the all-time peak on Steam for the Director’s cut was only about 6000. So the 1000 daily peak now is definitely fine.