I won't be able to fully replace it, I'm afraid. Not before communities gain the ability for their posts to not show up in high traffic feeds.
Some subreddits I follow have this set, but this is not yet implemented in Lemmy if I'm not mistaken. So a workable move to Lemmy for them is not possible at this moment.
YouTube has audio in Opus format@~150kbit/s. Opus is a much better format than MP3. Almost all audio is completely transparent at that bitrate, where with MP3s, there are cases where audio is not transparent without using non standard >320kbit/s bitrates (a lot of content is transparent @320kbits/s though).
Now, sites/tools like the one you mentioned take the Opus (or AAC) file/stream from YouTube, and lossily re-encodes it again, probably to a file that is larger than the original, with at best the same quality, but probably worse quality. You obviously can't get better output than the input in lossy compression.
So, the disk space argument is weird if you can play Opus/AAC (should be playable on every device nowadays).
This is the valid part for why you shouldn't use YT-to-MP3 converters.
But there are also invalid reasons why people will tell you it's shit: