If it's just a drawing of a mouse, then anyone can create the same value by just drawing a different mouse. Of course, you can't because of the millions of dollars that have been pumped into that particular mouse. That investment is what the copyright laws are protecting.

I would say that if something doesn't get published when there is a guaranteed, government-backed monopoly in place, then it's even less likely to get published when there isn't.

I don't think it's common for stuff to go unpublished when no one can find the copyright holder. In that case, just publish it and wait to see who complains.