Then you've already lost.
The BMC needs to be ideally on a physically isolated network, or at least a separate one that has no route from the outside nor on the machine itself.
Anything that makes privileges escalation exploits more damaging is a real problem. I’m getting tired of how these are being dismissed as if admin access should mean that you can ignore any security issues. There are things that even admin accounts should not be able to change at the hardware level, or if they can they must be reversible in the future by another user with admin access.
> The BMC needs to be ideally on a physically isolated network, or at least a separate one that has no route from the outside nor on the machine itself.
This is good practice but it shouldn’t excuse poor security at the hardware level.
Supermicro motherboards also commonly default to having a feature that bonds the BMC network interface to one of the main NICs if you don’t plug a cable into the BMC interface. It’s common for people to be surprised that their BMC is exposed on their main network because they didn’t plug in a cable on the BMC NIC port at all.
It's really the "permanently" which is the design flaw. Boards should have a mechanism to recover from bad firmware, and the same mechanism is useful to recover from a bad flash.
Make the flash chip removable, or leave a JTAG. Or have a bit of actual ROM with the write lines not even connected and just enough of a firmware to be able to reflash the main one.
This is exactly the kind of barrier you want for something with so much power over the system, otherwise you're not much better off than where you started as physical access allows for quick swaps of chips.