Anyone who doesn't understand the efficiency of the Opus codec should definitely try encoding some 64 and 96 kb/s files for themselves and listen carefully. The idea is excellent because it brings the same quality, but requires less data, so people with either slow connections or low data caps will be able to squeeze more out of SoundCloud.
However, they didn't delete the old MP3s, so a browser extension (search around, you'll see it) has been made to enable playback (this also helps machines that, for some reason, don't have Opus installed). This also suggests that the move wasn't made due to a possible lack of storage. Nevertheless, it's strange that they didn't make a public announcement or give users the choice between the two codecs.