Hi Joost, the problem with assuming that all paths are accessible for bicycles is that you inevitably make paths that are in reality inaccessible, accessible. Before we made the aforementioned decision, we got a lot of negative feedback from users that were sent over a certain path that turned out to be only for walking. When someone tells this to us, we do the necessary edits in OSM, but most of the users don’t make an effort to write an email about the problem.
A good example is the Hellegatbos near Kemmel in West-Vlaanderen. All of these paths had no bicycle tag, but were almost all of them forbidden for bicycles. If we would have made the assumption that all paths were accessible, our routings would have used all of these paths for mountain biking (I already added bicycle=no here a few months back though). Other good examples are / were Claeys Bouuaert domein in Gent, Kasteelsite Heusden-Gent and surrounding areas, Galgebossen between Poperinge and Ieper, … All of these cases were already partly or mostly fixed by us, but only because we knew that this was incomplete by already having gone there. There are undoubtedly a huge amount of other places where this is the case due to which you can not assume that a path is accessible for more than just walking.
Another problem is that highway=path is a very generic value for when you don’t really know what else it could be and it is used that way in a lot of places worldwide. In those cases, the necessary tags almost always aren’t added and we therefore can not assume it to be accessible for everything