I think that touches on something, that concerns OSM tagging a lot: Some tags are introduced by people that care about a certain aspect, from a particular position, and later get used by people, that do not care about that aspect, having no connection to the POV of the tag creators.
Some might wonder, why Yosemite granite looks so much different than Lüsener Fernerkogel granite? (Fernerkogel is surface=rock;rock=gneiss;gneiss=granite; while Yosemite has surface=rock;rock=granite)
In the end, its all gravel? The challenge to replace paved/unpaved with something more specific a lost endeavour?
PS: How about surface=paved;paved=rock;rock=granite vs. surface=unpaved;unpaved=rock;rock=granite?