An adversary can usually only modify code that executes in the boot process if they already have root privileges, or if they have physical access. In either of those cases the game is already over anyway.
Encrypted disks saves you from an unsophisticated attacker. Also, full disk encryption enables the feature of using a power plug switch as a ”lockdown mode” button.
If you're not worried about physical access, then why would you encrypt your disk at all?