I am unclear why some people think that MP3s are on the way out.
In fact, without them, it would be likely to expand - if there was anywhere for it to expand to...
Is there an audio player on the planet that can't do them? There were a few Linux distros that didn't include them by default. The first thing most people then did was to download the"non-free" stuff. Now it can be put into the main section.
MP3 is not gone. If it does not get any bigger, it is because there is nothing to expand into.