The idea is rather that someone with a large FLAC library, who has already transcoded some of it to MP3 for listening on a portable player, has little interest in transcoding their FLAC files to Vorbis again. The battery life of an MP3 player, even with a ten-year-old battery, is still more than ten hours of continuous playback regardless of the format, which is more than enough.

The real limiting factor is the maximum size supported by microSD cards. If the player wasn't limited to 64GB, I wouldn't even bother transcoding.

Sure, there's no massive gain from replacing mp3 with Opus. For new files, though, mp3/Vorbis doesn't make sense any more.

It took more than 10 years for Vorbis support to become widespread. It will take a few more years before we can hope to play Opus easily everywhere.

We are at a stage where current solutions are just good enough in most case. Change is therefore becoming increasingly slow.

Even for music streaming, many services continue to use MP3 and AAC.