Continuously assessing & evolving an executive leadership team is vital yet difficult.
First, people aren’t necessarily perfect packages of capabilities that meet every specific need. Often, people, even top managers, excel at some things but are not as competent in other matters.
Second, leaders may also be seemingly difficult to replace. Just because you are not satisfied with a current member of your team doesn't mean it will be easy to find an appropriate replacement. Nor does it indicate that the alternate will necessarily be an upgrade.
Third, there are learning curves to be concerned about. Someone new has to learn a new company role, and it might take some time before that individual is as proficient at day-to-day tasks as the predecessor. Moreover, a new leader also has to work with a new team, and earn the respect of other stakeholders in the organization.
So, the situation often comes down to the following. Is it a larger problem to keep somebody you should not have? Or to replace someone you shouldn't have?
Often times, there are no precise answers to such questions. It involves a matter of judgment.
This is one of a few key reasons why leadership is difficult.
@RayBordogna opines on "strategy" concepts.