This all seems like a lot of complication to solve an edge case. As far as I can tell, apart from some arbitrary notion of tidyness, the main advantage of this proposed new tagging scheme is that it provides a way to tag “dismounting is recommended but not required”. That’s a nice-to-have but could be solved in a way that doesn’t involve deprecating a tag that has existed from the earliest days of OSM.
“by far the exception” includes over 100,000 miles of footpaths in England and Wales. Big exception. I don’t know how many miles of “dismount recommended but not compulsory” paths there are across the world, but I’m guessing it’s less than 100,000.
As always, this is going to break a large number of routers which currently rely on bicycle=dismount. If you are proposing this, you also need to propose an outreach programme for data consumers, and commit to doing that outreach yourself (or identifying people to do so). It is not sufficient to just write something on an obscure wiki page and expect data consumers to magically discover it.