Also security engineer here. You're thinking too small and in the wrong direction. You're not protecting against 2G downgrade attacks, you're protecting against an attacker who has a whole library of things to try on you to see what works.

Lockdown mode is intended to protect against sophisticated actors, the kind who will buy 0days for six or seven figures, roll it into a new version of their product and sell it to governments.

Lockdown mode blocks the riskiest parts of the platform. For example, iMessage no longer automatically unfurls links, because anyone can send you a message and potentially send an exploit that your phone happily triggers without you doing anything.

You don't get to pick a la carte because Apple wants the feature to be effective and simple to enable.