These days it’s common to do firmware updates to address small issues or even support the new CPU that was launched after the motherboard was manufactured.
I could see manufacturers adding a write-protect physical switch for those who want it. Make it opt-in and toggleable.
Using the method you talk about would mean that this kind of update wouldnt be possible, 99% of users would never toggle with a switch to update firmware.
This would be a huge burden in the server world too, to unrack flip switch, update, revert switch re-install.
I assume you mean specifically motherboard firmware updates, because firmware updates are actually pretty common, for most server grade motherboards vendors ship updates about every other month[1].
1. https://git.kernel.org/pub/scm/linux/kernel/git/firmware/lin...