I think having multiple ECUs over time is a cost savings mechanism. You have a module that you can use in all your models because it works.

If Rivian wants to add a new feature that doesn't 'fit' on the AIO ecu, then what? Replace it and re-certify ALL of its functions?

A good example is the ABS controller which is usually just a Bosch COTS ECU.

You wouldn't even want to implement that for yourself.