But the difference is just a convention, no? One could very well add pathway=shared_use to a highway=path + foot=designated + bycicle=desginated" just as you could add pathway=scramble to highway=path + sac_scale=difficult_alpine_hiking. And you can at the same time remove highway=path from scramble much earlier than from shared_use in some distant future (if ever). So this would be much less disruptive (but also probably slower) then introducing new highway=* tags.
For people who were not aware of the busway controversy, these are the rabbit holes:
This is indeed important precedent. For me it is another reason why it would make sense to break highway=path into a new namespace (pathway=*). I also think the problem of busway is partly its definition - “A dedicated, separate roadway for buses” seemingly would includ short links meant for buses only (from a bus station to a road, diversion to a bus stop etc.) but further down the road one learns those are not actually busways. Somehow the assumption is busways are long, but that is not mentioned on the wiki until one gets deep down. It certainly makes the case that the differences between individual pathways would need to be thought through. (one advantage is that since cars require complex infrastructure with links overpasses and whatnot, ways that are not for cars are much simpler in their design, possibly making ducktagging easier).