FWIW, I think the policy is deliberately open-ended so as to be future-proof, but I doubt emails are an important source of advertising data today, so I think it's likely that Google doesn't rely on it that much anymore. Most sources of legitimate (i.e. non-spam email) that are useful for advertising -- e.g. online shops and the like -- already track you and show you ads, and Google is already deeply embedded there. Millions and millions of personal accounts are an useful strategic asset to have but I think there are better sources of data.
I completely disagree with the first part of your assertion. Email is still the main medium for all organizations, especially private companies taking your money for something, to communicate with you with plenty of details. Be it ordering some product online or booking a flight or other travel ticket or ordering a service or anything else.
The richness and amount of information conveyed over email pales in comparison to SMS notifications. So email is still a treasure trove of what people are doing and have done.
That's true, but all these places already track the living hell out of you. Even the newsletters they send over email have tracking information. By the time they've sent you an email after your first purchase, they know everything they need to show you relevant ads (in fact, that's probably why you made the first purchase...). I doubt bulk analysis of emails can show anything that is not already known way before the emails got sent.