I think I misinterpreted the keysmith README as “please migrate to quill”.
I also didn’t realize that both tools seem to support using an existing seed phrase to derive a key.
As you pointed out, keysmith can use that same seed phrase to derive multiple keys, which might appeal to people who have already securely backed up a phrase.
I did find this issue about certain phrases not being supported though: compatibility with identity.ic0.app seed phrase? · Issue #23 · dfinity/keysmith · GitHub