Plenty of people still use mp3 files. It’s not representative of the average person, although the average person isn’t going to be using this tool, but the large(est?) music tracker still accepts uploads in flac and mp3 format, scene releases are in flac or mp3. For a broader example, when people want to rip music from YouTube, they google stuff like “youtube to mp3” and if you google “download youtube music” top results names like “convert youtube to mp3” and the results not including “mp3” in the title offer downloads in mp3 format. It may not be technically the best format, but my parents know what an mp3 is, they have no idea what aac is.