> there is no way they could monitor conversations in the car for commercial purposes (at least in Western countries)

people used to feel that way about search queries, email (gmail) and IP laws (LLM training).

> What is exciting about monetization like this is the possibility for rides to become very cheap or even free. If my dentist offers free rides to the office in return for my loyalty, I'm quite happy to take that.

this won't happen. alphabet will collect on both ends.