The overhead lines represent the edges of the top empty opening. The threshold is actually another matter. I am in full agreement about the complexification. Though there is the argument that empty openings are not necessarily simple, it seems that should be the default, with the complexity to be added as necessary.
I'm not sure what the reason was, but at some point the "empty" openings ceased to be just simple holes in the wall and became library parts. This may have been necessary for compatibility with the introduction library part subtypes (in v8?). If so, it was unavoidable since the parts would be truly impossible to manage without subtypes (instead of just difficult).