No conspiracy theory, sorry. Thing is, you don't even need one.
I am very aware of people being stupid on the internet. However, it can also easily be linked to all the information they are gathering about you. Browsing websites, sending emails and so on aren't things you should have to wonder about whether you should be doing them or not. (except in cases of spam, and other bad things, but I'm aiming at "normal" usage)
Actually, they have done so yet. Gmail collected keywords from both incoming and outgoing emails and used them for advertising income. You can have your own opinion on the whole outgoing email thing, people who use Gmail potentially know this might be going on and could be okay with that. However, incoming emails are different, because the sender never said agreed to the Google terms. Therefor, those emails shouldn't be scanned nor used for income. Not sure how it's now, but this has actually happened.