Apple does seem to have grown into a place where senior people leave after a few years (either on their own or Apples accord). Is this because having a senior

Apple does seem to have grown into a place where senior people leave after a few years (either on their own or Apples accord). Is this because having a senior Apple role gives you a lot of opportunities and/or you can be sidelined or sacked if you screw up or fall out of favour? I'd say its better to have the '2nd best' designer for a longer period that having the best available every few years that takes the product in different directions and confuses the customers.