maybe it would be fine for them to put stuff on mobile platforms as I don't really think they are directly competing with Nintendo's core experience. Playing on the Wii U is epic, playing on an IPhone is a nightmare you will regret for the rest of your life. These are two completely different experiences.
However everywhere else I am against. First off Nintendo makes more money off of selling their games on their consoles in comparison to selling them on other consoles. Secondly would Nintendo's quality still be as high if they went third party? I know damn well Nintendo has about a hundred super senior developers who have been working with them since the stone age and are just waiting to retire. They haven't because they like Nintendo and they like what Nintendo does. If Nintendo went third party, I bet half of them would retire. Quality would drop like a rock, and Nintendo franchises wouldn't sell as well. Add 2 and 2 together and they would probably make less money than they are now.