Perhaps, but there’s a security angle to this as well. iBoot is stored on the internal SSD. If you want to bypass the internal SSD entirely for the boot process, you must now trust that any external drives plugged in have non-evil versions of iBoot installed, as the Mac will now have to go to every external device and say “can you tell me how to boot?”. If the internal drive is the only place to get iBoot it makes the security strategy way easier (since the only way to write to it is via DFU connected to Apple Configurator). We know only people with the keys can modify the internal drive, so we don’t have to keep certificates or something else around to verify it is trustworthy. Who knows where the external drive’s copy of iBoot is from.
You could make it so there’s a break glass copy of iBoot on another chip, but then that’s something they can’t update if there’s a bug.