Sophisticated general counsel do what must be done to have capable successors. Ideally, when the GC retires or leaves, the law department has one or more lawyers who can put on the crown. Internal succession is the best, I believe.
If there are no capable or acceptable internal candidates, the company must bring in someone from outside – a riskier decision. However, if the company realizes that there must be significant change in the law department, an outsider is more likely to be able to make major changes.
My conclusion: Groom internal successors, at all levels, but especially for the general counsel position; favor outside successors if the law department needs substantial improvement or redirection.