I really don't think the possibility of ViewRanger or its ilk ceasing to operate is much of an issue. There are so many other options if it happens.
The underlying issue is the strategy of using stored mapping in the first place, whether it be protected by proprietary software or no. Like paper maps, digital maps are out of date even before you buy them and get more so as time goes on. This may not be too much of an issue in more remote country where the only thing likely to happen is a new access track for the shooting fraternity or the felling of a forest that you intended to walk through (both have happened to me). However, in more populated areas, roads and houses get built, paths - both official and unofficial, get closed, moved or diverted. woods disappear and so on. The greater problem I suggest therefore, is access to current data to work out where to walk.
This is more sussamb's territory than mine but as I understand it OSM mapping is updated by its users so it probably provides the most current data and shows paths that people actually use whether they are on the OS map or not. Moreover, they are freely available to anyone at anytime so do not need to be stored. While OSM maps are perfectly OK for navigating, they lack the richness of information of OS maps. However, the OS attach higher priority to certain classes of information than others so while the road network may be fairly up to date, the underlying information can be years old. If you use the OS mapping app, you will at least have access to the latest version of their map rather than the version available from the product you are using. The OS app allows maps to be downloaded for offline use but once they are used they can be deleted until needed again. ViewRanger has a similar option for using their own maps offline but you have to pay for premium mapping such as OS so you are unlikely to delete it.
So what to do? Personally I use the OS mapping site to devise a route - it has a standard map, an aerial view and the OS leisure map. I refine that route by looking at open sources such as GoogleEarh or OSM. As a Garmin user, I still have to fall back on map tiles when I am out and about and I use OS mapping because I prefer it. However, I do not treat the map as gospel, instead I follow the route I have prepared, even if the OS map says there is nothing there. I usually have to same route on ViewRanger and the OS mapping app with the associated OS or VR map stored. When I am done I outload the trace and delete the phone maps.