There’s no definitive answer to this question apart mapping each section of way split to represent the scale as precisely as you can or want or remember.
There will always be a micromapper following your steps, anyway, and you won’t be able to argue against their photos.
Then let the data user choose if they show an average or a worst case scenario as they should.
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I added a summary of this discussion to the wiki: Key:sac_scale - OpenStreetMap Wiki
Problems, foot_scale should avoid:
Here a T1 route, according to Schweizer Wanderwege - Zwischen Pilatus und Bürgenstock - key location - https://www.schweizer-wanderwege.ch/media/cache/public_large/3036390997.jpg
Here a T3 route, according to above - Panoramawanderung vom Wildspitz zum Zugerberg - key location - https://www.schweizer-wanderwege.ch/media/cache/public_large/b5b535946616b31e3563a4ba82d959b37dc5f479f16b62219a80aa81c23d33c6.jpg
I do not know, who does the grading, but I think, it must be really hard to get right.
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Why shouldn’t we simply call this tag difficulty=*?
trail_difficulty=* if anything, like trail_visibility=* (though that would be pain to put in in JOSM). Just the transition pains, otherwise I do agree we should have something universal.
foot_scale (or foot_difficulty) does not imply anything for bike or something else people like using on those trails in the years to come.
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Exactly! And we want something specific to foot travel as the difficulty of biking on a trail may be completely different.
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So, you’re saying that
Includes climbing pitches up to UIAA grade II. Severe exposure. Difficult craggy terrain. Glaciers with high risk of sliding.
doesn’t tell you anything in case you want to ride your bike there?
You’re better served with mtb:scale, it goes up to 6!
Or you can guess from a hiking scale, but don’t complain it’s not suited to bikes afterwards 
We had a thread about it: Key mtb:scale useful for pedestrian routing?
leading to this passage in the wiki: Key:mtb:scale - OpenStreetMap Wiki
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For those following this thread, voting is currently underway on a related proposal: Proposal:Add strolling to sac scale and some further refinements - OpenStreetMap Wiki
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I don’t know what to feel about the proposal being voted. On one hand I love the idea of having a “strolling” value for a difficulty tag; it has lots of potential to clarify things with route=foot vs footway vs foot=yes. On the other hand, I am very receptive to the argument that naming sac_scale something that is not the SAC scale anymore may create confusion in the future.
Would initiating a walking:scale create less, or more problems?
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Confusion in the future you’re worried about, eh? 
OSM’s sac_scale already uses different descriptive words to the Swiss Alpine Club’s, and some SAC considerations are handled via other OSM tags (as has been pointed out previously by several people on numerous occasions). A “rename” to “stride ability and complication” as suggested elsewhere would only reflect the current situation.
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I’m in two minds about the proposal being voted on.
I thought foot_scale was promising, a real opportunity to introduce a scale developed by mappers and for mappers, designed from the ground up to be able to cope with the whole range of ways that can be found in OSM, as opposed to a scale (sac_scale) that was designed to classify hiking routes in the Swiss Alps, and haphazardly adapted for use in OSM because it seemed like a good idea in 2008.
On the other hand, convincing mappers and data consumers to adopt a completely new tag over the established sac_scale tag would have been very hard, so maybe it’s a more pragmatic solution to just accept that sac_scale has become a generic tag for the difficulty of hiking paths, and pull elements of the foot_scale proposal - like the strolling value - over into sac_scale
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I’m told that there have been successes of that approach.