People probably won't like this, but I want to point out that Recall from Microsoft tries to do a little bit of this. Apparently, the specific implementation of that product is a spectacular privacy disaster. Which actually may not be an accident -- it is probably not simple to handle the privacy well for a personalized search engine (again, even though Microsoft made a lot of obvious mistakes), and you probably want to ensure that the data you aggregate do not end up being sold by a third-party. Still, you need to build a viable business. That's hard.