BIP39 is used to derive BIP85 child key. I think it is the only option if using a single backup and index based derived key sets including BIP85 32bit hex keys used as nsec. It is very convenient if proper key management is required.
Discussion
I don't understand this condition:
> if using a single backup and index based derived key sets including BIP85 32bit hex keys used as nsec.
But if I could get away with ignoring it, I would only quote this part: "I think it is the only option" and then I would say "it's clearly not the only option because bitcoin core doesn't use bip39."
Sadly, I can't say that without doing an injustice to what you said
It's just a question of convenience. You can obviously use something else but having a seperate key backups for every thing becomes ineffective, especially if you're using steel backups.
Bitcon started off with address-based backups, it doesn't mean it was the most effective approach, it was the reason why folks build better solutions.