> (OTOH there certainly are languages with a tu/vous distinction where children were expected to "vous" their own parents – not sure how much of a thing it is these days).

It's interesting that in Viennese German (my German is terrible but I do at least try) it seems like the informal form is the default, in a shop I get asked "Braucht du hilfe?" rather than the formal "Kann ich Ihnen helfen?".

Maybe this is what they mean when they say people in Vienna are rude, but coming from Scotland using informal language even in fairly serious settings just seems comfortable and normal.