Well, I hope Nintendo's been planning for "life after" their older designers (not just Miyamoto) for quite some time now, because otherwise they're in real trouble. In any case, I have the utmost respect for Mr. Miyamoto. He's designed some of the most memorable games I've played in my life, and he's been the spearhead for so many other beloved titles. At Nintendo's annual press conference, he's consistently the only guy on-stage who appears to actually have a pulse. I'll gladly look forward to whatever smaller titles he wants to work on until his actual retirement. But when he leaves, I want whoever takes over from him to not just try to maintain the status quo. We're getting to the point where the new generation of great Nintendo designers have to come into their own.
Among many other problems, the Japanese game industry (especially) seems to have something metaphorically akin to an inbreeding problem: the old men who started the company train the next generation to make the same games they made. Then, when the new generation takes over, they continue under the direct approval of a committee run by the old men who trained them, who direct them to continue to make the same kind of games. I feel there's a lot of passion and creativity that gets lost in that system. I don't want to see that when we start losing these legendary designers at Nintendo. I want to see what the passion and inventiveness of a new generation can bring to Nintendo.
Until then, I'm interested to see what Miyamoto can do when he's not under the pressure of keeping an eye on half a dozen decades-old Nintendo properties at once.