The users producing most of the value that made Reddit interesting when it was a small website that had a lot to prove have probably already left. The thing is though, it's now very well-established. Reddit shows up extremely often when you Google it, and undoubtedly there's a lot of people (possibly a majority) who will just kind of shrug this off after deciding they'd rather suck it up than be without something like Reddit for a while.

But while the ad revenue is probably fine, the API revenue is definitely what they think the cash cow will be, and practically speaking they could lose 75% of their users and still probably not lose a whole lot of value just due to the sheer amount of historical data they have (especially if they start selling deleted content.)