Although I am against further fragmenting the =path category, I could see a use for the =pathless case for the most pathological cases where people still insist on using the path value. And though I can see that highway=pathless or path=pathless sounds oxymoronic, to me e.g. the quite popular classification of =unclassified appears equally paradoxical. I think @Minh_Nguyen 's point is on the nose:
The pathless key also addresses the extremely wide use of the =path-value, on the extreme end. As I have proposed ad nauseam, one possible solution would be to add more descriptive tags (I propose at least surface, smoothness and width estimates) to categorize the dizzying variety of paths. I can understand most of the criticism to this idea, but not all.
The point (to mirror @Minh_Nguyen, and address the current discussion) is that the lower we go in the hierarchy of the highway= key, the less physical properties can be truth-preservingly inferred from the value of the key.
In Europe and North America (at least), most motorways are e.g. paved with asphalt/concrete. cycleway and footway are based on legal/suitability criteria, but their surface value can no longer be inferred (perhaps only that it is probably not in the =ground superset). Most people use paths for the absolute pariah category of ways. Therefore next to no physical properties can be inferred from the value alone (except that it must be somehow visible, hence the need for the pathless alternative).
As I pointed out on another thread, this is one argument to treat ways tagged with only a highway=path as near inpassable routes. If the data consumers would adopt this interpretation and start to drop—or severly penalize—them in their routing, this might solve the problem of motivating people to add the descriptive keys. It is one thing to use a =1 m wide path with =grass or =clay surface, and quite another to walk or cycle on a =2 m wide path with a =compacted or fine_gravel surface of =good smoothness.
Lastly,
this appears to me to be a bad idea. The pathless case would have to depend on some kind of waymarker present on the ground (cairns, wooden poles or spray-painted rocks to mark the route).