It sounds to me that your positino is that if you do not have a law for something in Finland, it cannot have an OSM tag that is not super concrete.
Again, you do not treat roads and paths the same. What is the difference between the many highway road alues that makes it impossible to drive on one but not the other? If a highway needs to be distinguished by legal impossibility of usage, why do we have so many values? These high requirements are just not asked cars (they always seem to get a free pass or something :-/).
Again, as somebody said, somewhere you can bike and walk on a motorway. Just that something is possible somewhere on any type of OSM tag does not mean such thing cannot be categorized in OSM. I do not understand why it is such a problem. Yes, Alex Honnold can move in places where others cannot. but we are not going to put a highway=path over Fitz Roy because of that. Why the facsination with outliers? There are outliers everywhere (also in OSM). WE can safely ignore the 5-0% freakish usage.
By the way I do not ride a MTB. I just as a hiker want to avoid those mtb_single_tracks. I am not sure mtb_scale helps me with that, because it does not tell mu some way is not for me.
There seem to be this notion that there are only laws and complete anarchy. But you do not need to have laws for people to generally stay out of ways not suitable for them - common sense of the people is enough (ignoring th outliers, but why are they so important? We are describing a messy world, not some mathematically precise simulation where there could be only rules).