That's the point of persistent worlds. It's player characters living their own life and doing role-play between them with occasional animations by a DM. So if players want to start a mining business, they can (and many had already do that ;p).

The job of the DM (i'm talking in the case of PW, not in general) is to give to the player "tools" to help interactions between the players, make the world living, and some-time creating major plot-line within the module.

It's not just about 50 players fighting monsters all the day. There are quests with any fight involved, political intrigues and factions play.

In NwN, some of these worlds live since ten years, with player connected several times by week. But now, NwN start to becoming really old and a little wearying, so you can understand the hype in the NwN community with the DM mode of D:OS2 :'D