Fresh talent need mentors. Firing Miyamoto or Iwata resigning wouldn't solve any of Nintendo's immediate problems. It would just create a leadership vacuum that would be extremely difficult to replace, and would likely throw Nintendo's release schedules in even more disarray than they already are.
Who said anything about firing him?? He just doesn't need to be the guy that filters every thing that comes from the comoany anymore. He should be training and advising the new guys along with working on smaller stuff