In the Picasso example, if the you weren't selling your sketches before, then the potential copyright rights were likely not the incentive needed to create the work in the first place, practicing your craft was. Also, just because you don't have a copyright doesn't mean you can't sell it. In this use it or lose it hypothetical system of copyright, it just means that you can't prevent others from using and building off that work. I'll also note that even actual modern copyright treats unpublished works differently than published ones. In the context of the article about live-service games, your sketchbook example would be more like an unpublished game like 0x10c. I'll agree that there might be reasons to still treat published works differently than unpublished ones.

In your second example, again it would be about extra exclusivity rights you get from copyright, not anything you necessarily need to do for every slightly creative act you're involved in. If you were never going to assert copyright on those, why should there need to be copyright protections provided by the public?

In general, the way I was mainly thinking of this kind of potential requirement was for things that have already been distributed to the public but then no longer supported/sold. I see things being made part of the culture by being released but then becoming totally unavailable to be one of the worst things that can happen to creative works when we now have the technology and capability to preserve everything.

No, it's more coercive. If I own a bagpack, that means people can't break into my house and beat me up to get my bagpack. If I own a song, that means I can stop anyone else in the world from publicly singing the song at my leisure. One is much more coercive than the other.

Maybe, but property is typically an item, which you can't easily copy. Copyright is for items which are easily copied. So, it not the same. If you download a car it's different than stealing it, because original owner of car still has the car.