While “best guess” might work in some situations (a blog post about how wonderful OSM coverage is, for example), but I’d suggest that a router should only suggest a route that it knows will be OK. The access tag “best guesses” that you suggested above are just plain wrong in the countries I live in and visit most.
If a particular tag (such as highway=path) has such broad usage - even if you exclude frankly silly examples like the “scuba path” mentioned elsewhere - that it can’t be relied upon for a particular mode of transport without other clarifying tags, then a router shouldn’t suggest it.
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