That last paragraph lost me… Are you saying we shouldn’t create any new top-level tags (hello highway=busway) just because we’re afraid they’ll stray from their intended use?
Also, it seems like you missed the highway=motorcycleway proposal – probably because it doesn’t exist yet 
Following your logic, I might as well start a proposal to deprecate highway=footway/cycleway/path/bridleway/pedestrian and have mappers “stick to physical aspects” together with highway=residential/service/track.
Based on your reasoning:
- A narrow footpath leading to house fronts will be then tagged as
highway=residential + width=1 + access=no + foot=yes. - A narrow hiking trail will be tagged
highway=track + width=1 + motor_vehicle=no. - A pedestrian road would probably be tagged as
highway=residential/service + motor_vehicle=no - A cycleway would probably be
highway=service + width=x + access=no + bicycle=yes.