While there are some situations that are very time consuming I feel those who pursue a graphic field need to constantly keep up with the latest and greatest. osu! is not an advanced graphics application although there are some features that can help it along but does not change the fact. There are options that can accomplish what your asking such as slicey for photoshop under OSX or the newest Photoshop CC which uses generator engine that is fully scriptable which is what I currently use for osu!! assets. To be fair it is not the job of a game to be a graphic consolidator, at one point I was exporting assets by hand at their compatible resolutions but if you spend some time you'll come to find there are organizations that do this on an extremely large scale so I suggest you keep up.
I don't really see the downside to this? Unless it causes performance issues. If osu! can scale 1x images up to 2x (as it does) would it be any harder to scale 2x images down to 1x? In most cases where there are more than one potential version of something, osu will use whichever it finds, not sure why that wouldn't apply here.