I think it's a mixture of things.
Regarding players it's mostly due to us having such a strong roster so some players will eventually leave to become the focal point in another team and to play more regularly. In a majority of cases this is the only time we lose players we want to keep.
Regarding staff, some will eventually be cherry picked from other clubs and given dream positions which they can't turn down. This is because other clubs know how professional we are run and that we have a culture of success which permeates through the club.
In any very successful organisation, these types of turnovers will always occur but if the infrastructure is strong, then whoever comes in will usually do a good job as they gave the right support systems in place.
On the opposite end you look at a club like Utd and it doesn't matter who they bring in, they end up failing because the infrastructure and support systems are not there.