Well the truth of the matter was that sensory keys were added to the (security) kernel about the same time and this was because the factory made them much more important. Without segment keys that were manifestly read-only to the factory (lest the instances gossip among themselves about their respective clients via their shared code segment) one of several more complex or more expensive schemes would have been required:

Sensory keys are by no means a construct private to the factory. Any program can use them just as the factory does.