I agree with you but to be precise full disk encryption does not provide any benefit that is not met by encrypting user writeable mounts. Encrypting /usr, /bin, /lib etc does nothing in this situation.

The point is to catch the unexpected, just as this scenario was unexpected.

I can't think of a realistic situation right now that will catch you out, but with full disk encryption I don't have to.

Full disk encryption provides some level of protection against backdooring.

Under what threat model? (Backdooring is also beyond the scope of the initial doiscussion)