Open means that the owner of the device can replace the firmware. There's nothing not-open about requiring that the owner authenticate first.
It's only not open if the vendor, rather than the owner, is the only one who the device will permit to replace its firmware.
It means that a small minority of owners of the device can replace the firmware. Most people don't have the resources to do something like that.
Sure, only a small minority of device owners can actually make new firmware, but fortunately only one of them needs to make and distribute some firmware for everyone to be able to install it, if the device is open.
No, even if they personally don't want to touch the engine, they turn to knowledgeable friends/family or pay a professional mechanic to do the work.
This unfortunately-common view that most people should never even have the option to do anything technical is insulting and maybe even monopolistic (it bans 3rd party repairs/parts).
A better question is how to further lock the phone down in future updates so that Apple can't comply even if they were compelled to, something Apple engineers are no doubt looking into right now. The optimal way would be to require a device to be unlocked to update the firmware, which would require the PIN, which Apple would not have.