Yes, hillshading and contour lines are very important in the hills and mountains (and not interesting in many flat regions), but as they aren’t data we are collecting, for our showcase default map it isn’t necessarily helpful (one might argue that the data we do collect can be better understood with the terrain as a context, and I’d agree to it). I’d keep the default map “simple” (focused on our own data) because our strength is detail (where mappers are) and in detailed views, the commonly used (“globally” available) 30m or 90m elevation grids are not sufficient.
On the other hand I also miss hillshading for checking out walking navigation on the website or to browse with the terrain context, and the two bicycle oriented maps are not perfect if you do not want to focus on cycling (naturally). There is waymarked trails which is very recommendable and while not reachable from osm.org you can go directly to their site if you know about it (not sure if @lonvia could be interested in eventually getting a traffic boost from being integrated on osm.org home?) — the hillshading is a separate overlay layer that you can activate (could this be an option for osm.org?)