If it leaves or doesn't leave Google servers we can't really know but the facts are: -Google has this information -They are marketing company with incentives to monetize it

Imo it's enough to be a bad thing. We can't rely on good will of companies to fight incentives and money in the name of privacy of their users.

You don't have to rely on "good will". Even if you are super pessimistic "companies only want money", Google will never sell or give up that data to other companies. That data is literally Google's lifeblood. That data is why companies use AdSense to advertise with instead of Bing Ads. That data is Google's competitive edge.

Even the evilest of companies isn't going to sell off its competitive edge, that's just idiotic.

Also you are wrong, Google is not a marketing company. Google is a middle man company. Google matches marketers to consumers. THAT is Google's business. Google is paid to be a match maker, nothing more.

Until the NSA gets to it.

Or until, years and years from now, someone buys Google.

when gmail will be as quaint as a model-T