Extending your seed phrase would ensure that the adversary won't directly gain access to your wallet but it doesn't mean that you won't be susceptible to extortion or a $5 wrench attack.
Extending your seed phrase with a passphrase can save you from a $5 wrench attack through plausible deniability, if you load the base wallet with a small amount of funds, you may be lucky to deceive the attacker that it is the only coins you have, without them knowing that you have a wallet 'hidden' by a passphrase.