To have (sugar) or to have not (sugar)

That depends on the rum and the ABV. I often let cask strength rums with over 60% breathe for over an hour or longer so that the rum can develop its aromas. Therefore, I cannot confirm that a rum basically loses its aromas in the glass. There is certainly a limit, but if you don't swirl the rum in the glass too much, the contact with the air is quite interessting. I think if there is too much air in the bottle, the rum will change over time. First it may become a little softer, but then it also loses aromas. Especially rums with high conngeners and fine notes. These are often only subtle differences, but decanting can't hurt :). If the rum is drunk for years.