If the given wallet is BIP85 compliant then there is no need to write down all bunch of the child mnemonics. All of them are derived from a single master SEED which makes mnemonic management to be very easy matter. For instance in the case of HW backup one has to write down solely the master mnemonic and be safe forever. BTW, Passport 2 has such functionality as generate multiple child SEEDs from master mnemonic while Ledger Stax has not.
I don't like BIP85 personally, because it overloads the "coin" part of the deviation number. Coin is supposed to be used (per BIP44) to identify the coin network. As someone who's making a wallet library, this means that I won't be able to use BIP85 and multi-coins at the same time.
Why didn't they just use "account", which is also hardened? Is that not it's intended purpose?