Having the checksumming system unmodifiable would be similar to disallowing modification of speedometer and odometer. Important measurement instruments were always kind of a special case.
You could use this to limit modification of some crucial systems (ABS, airbags, etc. Those will always be one septate microcontrollers anyways because of their hard real time requirements).
Or as you said you could keep all modified cars out of public traffic, while still allowing experimentation on private roads. Both variations are more heavy-handed than I would prefer, but a lot better than the status quo.
You could use this to limit modification of some crucial systems (ABS, airbags, etc. Those will always be one septate microcontrollers anyways because of their hard real time requirements).
Or as you said you could keep all modified cars out of public traffic, while still allowing experimentation on private roads. Both variations are more heavy-handed than I would prefer, but a lot better than the status quo.