Well, same colloquial usage caveats apply to “path” as used in highway=path, don’t they? And still, it gets used with all different trail_visibility and sac_scale values. Also note that trails usually get trailblazed exactly because their trail_visibility is way below excellent. So much for precision of colloquial speech.
I would also like to note, that we have (although not quite at Humpty Dumpty level) some artistic freedom in use of words in OSM tags.
So we have shop=massage or shop=hairdresser which are not really shops, highway=bus_stop which is not a highway, waterway=fuel which is not a waterway at all etc. We have cuisine=heuriger which is not even an English word, or cuisine=brunch which indicates time of day for meal, and not cuisine.
Tags mean what we define them to mean in the wiki. It is nice bonus if it makes sense in colloquial (and/or British) English when you read key and value aloud, but it is not requirement at all. What the specific key=value means is what you define it means in a wiki.
Unless its name is hugely different (e.g. intentionally opposite) of what it means, such minor deviations from British English Dictionary de jour are not really a problem, as long as exact meaning is clearly explained at tag wiki page (and in proposal before that!)
So yeah, if we can agree on highway=demanding_spoor for example, we can define there that the actually visibility of that spoor is defined by extra tag trail_visibility=*.
We should also define there, what it means if trail_visibility=* is not specified: for example, that the trail_visibility=intermediate is to be assumed (or, trail_visibility=no is to be assumed, or whatever we agree on as most likely scenario. Or, we can even clearly define on that wiki page that no assumption about trail visibility may be assumed if it is not explicitly specified – what is important is that we be crystal clear about it upfront, before tag becomes official and starts being used).