I work at OpenAI and I'm happy to deny this hypothesis.

Our goal for the router (whether you think we achieved it or not) was purely to make the experience smoother and spare people from having to manually select thinking models for tasks that benefit from extra thinking. Without the router, lots of people just defaulted to 4o and never bothered using o3. With the router, people are getting to use the more powerful thinking models more often. The router isn't perfect by any means - we're always trying to improve things - but any paid user who doesn't like it can still manually select the model they want. Our goal was always a smoother experience, not ad injection or cost optimization.