> I see no reason to encode stuff into .mp3

for me it comes down to:

    1/ everything can play mp3s
    2/ audio quality at 320kbps is good enough for me
    3/ disk space is just not an issue anymore so I don't need more efficient codecs

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.

In a context of "converting youtube into mp3", "technically not the best format" is an understatement for what most likely is "conversion" of 128 kbps aac or opus into "320 kbps" mp3s (or whatever random bitrate). The least one could do is just listen to the aac/m4a or opus files without converting them yet another time.

Losssy-to-lossy is a bad idea. Getting mp3 off YouTube means very crappy experience, even if it is somewhat popular

Software that my parents or yours can use is definitely a good thing. Not everyone needs to chase the cutting edge.

That was my point but I feel like everyone that replied to me thought I was making the opposite. I think your submission is awesome :)

Most people know what an "mp3" is. Not many people know what an "aac" or "opus" is. Mp3 just works practically everywhere, but aac and opus - not so much.