The overwhelming majority of the audio files I have are .mp3.
I don't even know if my ".mp3 player" can handle any other format; it has trouble with a few file formats for sure, so I convert the files just to be able to listen to them. This is usually a once-and-done deal for me. I honestly can't tell the difference other than "does it play?" and I have decent hearing. Maybe if I were some purist with perfect pitch I'd be throwing fits, but I'll leave that for other people. The audio is fine. I don't hear the problem.
The concern is not a one-time conversion to MP3 but several generations of MP3s derived from each other, each one with more artifacts than the last. It's the same problem we experience with image compression. Lossy compression of a lossy source results in a lossier output. From a preservation point-of-view, it's best to have a lossless (perhaps compressed) copy and use that as a source for whatever arbitrary audio format we're using decades from now.
I don't even know if my ".mp3 player" can handle any other format; it has trouble with a few file formats for sure, so I convert the files just to be able to listen to them. This is usually a once-and-done deal for me. I honestly can't tell the difference other than "does it play?" and I have decent hearing. Maybe if I were some purist with perfect pitch I'd be throwing fits, but I'll leave that for other people. The audio is fine. I don't hear the problem.