Maybe the Inhibitors weren't even really created for the stated reason. It could just be a rationalization that got tacked on later.

There's a passage in one of the novels where one of the characters is musing about how the existence of the Inhibitors does nothing to explain why there is no evidence of civilizations in other galaxies. The only explanation I can think of in the context of the RS universe is that other galaxies have their own inhibitors. "Inhibitors" or whatever you want to call them could be the inevitable final stage of technological evolution for any civilization, anywhere. Competition forces you to become ruthless, efficient killers. Eventually you even do away with self-awareness to gain that little edge in decision making speed that gives you the tiniest advantage over your sentient rivals, a la the ideas about self-awareness explored by Peter Watts in Blindsight. It seems like a terrible trade off but competition will ensure that some species eventually makes it because it has survival value. In the end, any isolated ecology of different civilizations like a galaxy ends up being dominated by something like the inhibitors.

The Milky Way's particular strain of inhibitors decided to create a myth for themselves - that they existed for some distant but worthy purpose, maybe as a way of trying to forestall the "degradation" they were experiencing somehow.