It seems like he got lucky, and was good enough to leverage his success into more success and was good enough of a businessman to profit handsomely from the whole thing, but that's about it. I understand that he hasn't done anything on Minecraft in quite a while. His attempts at something new don't seem to have gone anywhere. (That weird space programming game from a while back got a lot of attention but never resulted in anything.)
So, maybe that was it. Notch is a one-hit wonder. And there's nothing wrong with that. I'm sure he's already contributed more to the world than most of us ever will. But that doesn't necessarily mean that we're all losing out on anything significant if he doesn't keep trying to make more big things.
And from the other side of things, it seems like Minecraft came out of one of these ridiculous idle amusements. If you had described Minecraft to people before it was made, they would have thought it was ridiculous. Oh, a volumetric 3D world with a 1990s-era renderer. Great. Sounds about as useful as a Doom map viewer written in Dart. Yet it turned into something huge. So if Notch does have another hit in him, it will probably come about the same way, through more strange messing about.