Poos aside, would we agree that if going for just a wee, it would be usual for guests to use the downstairs cloakroom, if one exists?
I maintain, that one of the key reasons houses have a downstairs cloakroom, is so that guests can use it and not need to venture up upstairs into what might well be considered the private bedrooms and bathrooms of the family. Of course if there is only 1 bathroom and its upstairs, everyone understands guests will go upstairs.
All this talk about pooing - of course guests do sometimes need to po whilst visiting friends or family, but most visits are just for a wee in reality. I do t think there is any need for guests to go upstairs when a loo is available downstairs in this case.
What about workmen who are visiting (assuming you allow them to use the loo - I know some MNers are funny about this and refuse workmen the use of the loo - odd and rude in my opinion) - would you expect them to use the downstairs loo or feel they could do upstairs too?
Perhaps I think back to days when houses with downstairs loos tended to be the bigger 4+ bedroom houses. The owners had a variety of tradespeople or daily help coming in, as well as visitors - the downstairs loo was very much to dilienate the areas of the house which visitors could access, which were separate from those for family use, which were private. If you go into a Vicarage for example, the Vicar's study and downstairs loo will be near the front door - allowing any visitors from the parish to have a meeting innthenoffice and use the loo, without need to go into the living room, kitchen or certainly upstairs.
So, I think the idea of downstairs loos being where guests go is historic. And personally, when visiting, I would use the provided cloakroom if there was one and only go upstairs for any reason if specifically invited. Perhaps people have become more relaxed and feel at home and more free to go where they like in the houses of others these days. I always taught my children to not go into bedrooms or bathrooms at friends houses unless specifically asked.....but I've had friends come here with their children and later found that a herd of children have been in my bedroom and my ensuite and also in my wardrobes - no sense at all that those might be private spaces - signs of changing times?