But imagine yourself up on stage in front of thousands of people, with 2 or 3 seconds to come up with an answer while you are not allowed to mention any unannounced games. What would you have said? This is hard and many of us would not have done much better than this on the spot.
The only "better" answer that I can think of would be a vague, general hint that Blozzard is working on more titles than those that are announced. But that would immediately be interpreted as "Diablo IV confirmed" and blown out of proportion.
The gaming community has in recent years developed a culture of latching on to smallest of hints, letting their imagination roam freely to create a kind of groupthink dream of what the project would be. And than they react not only with disappointment, but also with hate, anger and aggression against the developers when these dream bubbles are busted. This lack of moderation in the community is disconcerting and if we don't find efficient ways to dampen that, we will see this get out of hands.
Games are made by companies for profit. If they release something that you like, enjoy it. If you don't like it, move on. I don't understand any need to fuel hate to the levels we are seeing repeatedly these days.