I haven't used butter in sandwiches for nearly 40 years.
I'm English but lived in the US for a short time about 40 years ago. There, I happened to be in someone's kitchen and was helping her to make sandwiches. I saw that she was using mayo and not butter, so I did the same.
Upon eating the sandwich, I realised how much nicer it was than one made with butter. The added bonus is that mayo is so much easier to spread, especially when the bread is very fresh. Ever since then it's been mayo all the way for me!