Coming from a Gamekult interview with Shigeru Miyamoto...

Creating a game of the caliber of Super Mario 3D World takes a lot of energy and there is little chance that I could take the time to work on the next Mario. However, I want to get involved again and more deeply in the development of smaller projects, but that doesn’t mean it will be a modest download-only title or something like that. It is not impossible for me to work on a new game as the main producer in the future, but even when I’m not directly in charge of a project, I always look at what the teams does in terms of gameplay, and I trust them. Don’t worry about Mario!

Like we've said a million times before, there will come a day when Miyamoto leaves Nintendo. That's why Miyamoto has been grooming devs as the years role on. They'll have to take over for him one day!