I see this the same: It is not about defining defaults, instead it is about adding detail to the mappings.
Maybe editor presets to the rescue? They might pull in a number of keys, that mappers probably would never consider relevant for what they are about to map. Smoothness on a hiking path, mtb:scale probably not mapped by walkers But useful for users?
Pondering some such “presets”
- Sunday stroll
- Trail run workout
- Demanding mountain hike
Actually, I see that a bit duty of consumers, to come up with a list, what attributes they want, whether to render or to route something? Or can the OSM community guide them?