Except that it doesn’t, at least not necessarily. I’ve written about this on another thread so won’t repeat that here. designated simply means some sort of official designation, and in parts of the world this absolutely does have a different meaning on a footway vs. on a path. I don’t know (or much care) what designated was intended to express, but it absolutely can be used and does express these kinds of real-life differences.
Sure, and like I said, I’m not categorically against a footcycleway. But let’s take your idea further. Would you be okay with a footcyclewaysegregated (and, of course also cyclewaysegregated & footwaysegregated) top-level tags? If that rubs you the wrong way, then surely your reasoning does extend to mopeds? Mopeds are allowed on some cycleways, so how about footcyclemopedway (and, naturally of course thus also the cyclemopedway, cyclemopedwaysegregated & footcyclemopedwaysegregated) tags? Also service vehicles are allowed on some cycleways, and we haven’t even started to ‘integrate’ the surface tags: footcyclemopedservicecarwaysegregatedasphalt … I hope you get my point about the power of combinatorics mentioned earlier. Where, in your opinion, would be a ‘natural’ or ‘obvious’ place to stop, and why?
Also, at least some road use categories form natural hierarchies. I’d say that highway=mtbway suggested by @julcnx earlier, fails for this reason. An MTB is a type of bicycle, and therefore exists on a hierarchy. It is therefore much better to use the mtb:scale tags for them. I won’t take my roadie to anything other that mtb:scale=0 tagged way (or one that doesn’t have an mtb:scale at all), but might ride my commuter bike on a short stretch of mtb:scale=2 tagged way if it saves me time. Also, deciding between a path and an mtbway invites edit wars, but it is easy to have an additional tag for MTBs that also allows gradations (at least in the FtR world, where bicycles are legally allowed everywhere).
I thought we have highway=track for those:
The tag highway=track is used for minor land-access roads that are not considered part of the general-purpose road network […] Track roads are used by four-wheeled (two-track) vehicles and often take the form of two wheel tracks in the ground