Also, jumpers are cheap. Just set the jumper to enable writes, and download the update.
Are you happy with it being unknowable which of your appliances are compromised or not? Would you pay $1 more for a disk drive with firmware in ROM? I would. If you were running a banking system, would you pay extra for code in ROM that cannot be compromised?
As for IoT devices, I can't imagine average Joe or Jane prying off their smart whatever, destroying the pretty plastic casing it's hidden in, and manually setting jumpers to flash firmware.
I don't think the current scheme is working very well. Even worse, there is no way to tell if your appliance has been compromised or not. There are a lot of companies that care whether their machines are infected or not.