My initial thought was that making locked playlists read only was a good idea. The first issue I ran into was the format change I mentioned, but that seemed like a reasonable thing to have to update the playlist for.
However, I just realized/discovered that this is causing playlists to break even over basic tag changes. I changed the title of a track, and it disappeared from the playlist because the file name changed to match. That makes locked playlists a lot more work to maintain, when the basic idea is to protect the playlist from accidental changes like a deleted files. At least for MusicBee's own playlist format, making it read only doesn't seem like it would make much difference anyway, since it wouldn't be edited externally. And for basic .m3u files I'd think if someone knows their way around playlists well enough to edit it manually, they'd have to be making an intentional change.
So maybe read only is overkill.