In your case, (I presume) you are both the owner and the only user with physical access, and "you control your own device" is unique.
In my case I'm the owner, but other people may have temporary physical access, and the identity of the "you" is therefore somewhat ambiguous. As owner I want protection against one of the possible values of "you", see?
If the owner of the device flashed their ROM, they want updates. If someone other than the owner flashed the ROM, the true owner of the device is screwed anyway. Blocking updates won't save the true owner. Instead, this seems nothing more than lockdown for the sake of control.
It was written to flash by untrusted software, it may have been modified on the way such that it thinks it's unmodified, such that the signatures match when it checks itself, but actual execution uses a modification by that untrusted code.
It's a bit like that paper about modifying the compiler. https://www.win.tue.nl/~aeb/linux/hh/thompson/trust.html
From the owner's point of view, being able to update the ROM is the manufacturer's statement that nothing done by a user with physical access survives.
I accept that the manufacturer's OTA update is intended to be monolothic, is desigend to be monolithic, but what assurance do I (the owner) have that the software that was flashed by a physical user actually flashes its replacement monolithically? That it leaves nothing behind?
EDIT: on further reflection, it seems possible to design a phone that provides such an assurance. That any monolothic OTA update actually has to be monolithic, even if untrusted software is in control of the main CPU. But I wouldn't want to bet that any/many/most phones built today actually offer that guarantee.
Yes. I can't see any technical reason why Google disallows factory resetting my device without re-flashing a ROM.