You don't have to wait for the worst to happen. They don't have to blackmail and/or threaten you. They don't even have to abuse the data. What they have already gives them informational asymmetry over you and other users in aggregate - and that's what matters to them. By having access to your search history, documents, emails, spreadsheets, pictures, text messages, blogs, videos, your web page analytics, visitors, purchases, location, friends etc etc they're always guaranteed to be one step ahead of you. That ensures predicting with satisfiable accuracy, say, what you'll buy next, what you'll visit next, where you'd click next, where you'd go next, or, in general what you'll do next. This gives them a clear advantage in terms of controlling a good part of your online life and in turn a good part of your real life. If you feel that's something you can live with, that's fine. Older people recommend not putting all the eggs in one basket (even Google apparently listens to them by diversifying immensely). I am sorry if this may sounds a bit unrealistic/gloomy to you but there's never been an entity in the history of the world that has not abused the power that was given to them.

"Are you worried about a leak?"

Nobody can't argue it can't happen, why take a risk of having our entire search history in one place.