It might be being a tad simplistic to believe that if God showed up everyone would suddenly believe in him. There's always going to be a large number of doubters, who won't believe what they see, that sort of thing. Even in the Bible, when God's visits were infrequent but apparently fairly spectacular, it seemed that it didn't take long for people to stop caring, or even not to pay attention at the time.
Playing "God's Advocate" again... to some extent. But arguably little would be achieved by yet another demonstration of His power and then nothing for several generations, other than yet another bounce up in the number of people who worship Him, followed by another lingering decline.
Not that this really supports the idea of God much, for me, because then -- what can He actually do? It's like a street performer, who can be as wonderful as anything but most people just aren't interested.
I've recently taken to wondering how Heaven can possibly work if we're in it.