I think this is only true under the assumption "voice recognition (and transcription) is solved". I think most would consider this assumption to not be true. If it is not true, then there is value to that voice data, as it can be used to help train.

I guess another assumption could be "they have more than enough voice data to train any future network improvements." While I feel they have a lot of voice data, I am skeptical to say that they wouldn't view more data as useful, or at least potentially useful.

I cannot think of a compelling reason for why they would stop collecting data all of a sudden. Can you? (Serious question, I don't mean to sound snarky)

I think you're completely right. I interpreted data collection here as a mining activity for ads preferences. Currently, they fully collect the voice recordings and only seem to provide options on whether the data is connected to your account. I don't see any option to "forget what I said after you've responded."

You might be waiting a while. The problem you describe - with GPS - is an acoustic modeling problem, and a very difficult one at that. Cars come in many shapes and make lots of odd noises and various speeds, not to mention these devices are mounted in many places in relation to the speaker.

The voice recognition on Android Auto works fine in the interior of every car where I've tried it. The manufacturers include well-placed microphones dedicated to the purpose.

Hopefully those abilities and integration will filter into open source implementations. Right now, it seems like AM is owned by the big players and will be for the foreseeable future.