Even a successful management team needs their talent pool rejuvenated at regular intervals. Move too slowly and inevitably performance declines . Move too fast and the instability and loss of expertise leads inevitably to declining performance. Building on success requires decisive often ruthless intervention.
You can only hold on to your more experienced and talented individuals for so long. Their skills match their ambitions and they are the targets of head hunters who are quick to tell them of opportunities. They did a good job for you, their ambitions can’t be fulfilled where they are so they move on and up. There are others whose energy levels have dipped, who seem less committed and ambitious than in the past. You need to replace them or at least give them some serious competition to see if they can rediscover that drive and enthusiasm.
You have some young talented managers who show great potential and you certainly want to give them opportunities to gain the necessary knowledge and experience but you can’t rely on them to take on the full responsibility when their still learning, still a little naive and coming up against some complex challenges  for the first time. Throw them in at the deep end before they are ready and you may shatter their confidence and set back their development.  So you need to recruit some talent, with a bit more experience  but who have the hunger and stamina as well as the creativity to rejuvenate the organisation.
In theory this is an on going process. The organisation invests in developing managers at the same time it has to replace experienced and talented individuals who move on. Ironically the problem arises out of success. A stable and talented senior management  team takes the organisation into a period of unprecedented success. The effort from the leadership and board is to retain this team. To the extent they are successful no new talent is recruited and those coming through the management development process don’t get the opportunities and experience they might have expected. The implications of not rejuvenation the talent pool are often masked by the collective expertise the team can draw on but gradually it becomes clear than the organisation has been coasting.
Organisations need to creat opportunities for young talented managers to progress into senior posts. The organisation needs to recognise the need to recruit experienced and energetic outside talent to push the existing senior management team and to provide ready made cover for when experience individuals leave. Most of all the chief executive and board need to recognise the danger of leaving it till things start coasting before they get ruthless in rejuvenating the senior management team.