I think the key difference is in the motivation of the player. Many people have hobbies that one might call "grindy", consider how much time and effort goes int

I think the key difference is in the motivation of the player. Many people have hobbies that one might call "grindy", consider how much time and effort goes into gardening for example, but because they're not getting paid we don't call this work. For me it becomes work when the motivation to keep playing shifts from intrinsic (sense of achievement, community, completion, etc) to extrinsic (money).