In terms of just the media file format, Ogg Vorbis and WMA also have better bit-sound ratios than MP3.
The issue is compatibility. Apple’s DRM was ensuring that iTunes Music Store customers had to use Apple’s devices to play back their purchases. Getting rid of DRM but keeping AAC accomplishes the same thing.
I’m skeptical that there is anything to gain at all WRT compression ratios. Spreading file formats is nearly impossible, adding disk capacity is really easy.