Let’s just say it’s a lack of imagination on our part: I don’t have a BMX rider in mind when mapping urban sidewalks, and you probably don’t have a city bus in mind when mapping paths in more rural areas.
In most jurisdictions, the law similarly only accounts for the most obvious troublemakers, not for the most extreme stunts that someone might be inspired to pull based on OSM forum discussions.
Where does this leave us? If we aren’t “tagging for the router” in some abstract sense (as opposed to mistagging for a specific router), then what are we even tagging access for?
Emphasis on “and has been with us ever since”. A non-legalistic interpretation of access tagging has always been present in OSM and continues to be present today; otherwise we wouldn’t be having this discussion.
I never bring up old versions of wiki pages to insist that OSM tagging is immutable, only to point out that a particular interpretation is not novel, in case there’s a misconception that it is. Conversely, all too often I see people point to the latest version of a wiki page as gospel, ignoring the fact that anyone can change the wiki unilaterally. It’s up to us to be vigilant about anything that has ever gone into the wiki and make sure it matches the community’s needs and desires (and what’s actually in the database).
If this is your point, then I think your proposal needs an adjustment. Repurposing discouraged for the no values you find problematic just shifts the problem to someone else to deal with, which isn’t fair.