There is a slightly odd debate that happens around senior appointments. Internal candidates are described as the safe option. External candidates are described as bringing something new.
Neither is necessarily true.
I've met internal candidates who would completely change an organisation and external candidates who are a very expensive version of what the business already has.
The better question is: what does the organisation need, and who is best placed to deliver it?
Define the role as if nobody currently worked there
This is harder than it sounds. If an internal successor is well known, the role can unconsciously get shaped around their strengths. If the Board has already decided it wants change, the opposite can happen and the specification becomes a description of everything the existing team supposedly lacks.
I'd separate the two decisions.
First define the outcomes, scope, relationships and capabilities the role genuinely requires. Then assess people against that definition.
Give internal candidates a proper assessment
An internal candidate shouldn't have to compete with the mythology of an external market nobody has actually tested.
Look at evidence: what have they delivered, at what scale, in what circumstances? Where have they operated beyond their current remit? How do they handle ambiguity, challenge and senior stakeholders? What would need to be true for them to succeed in the bigger role?
Assessment tools can help, but they shouldn't replace judgement.
Test the external market without committing to a search
This is the bit I think organisations underuse.
You can map the external market before launching a full recruitment process. Identify credible comparators, understand likely candidates, test remuneration and get a sense of what additional capability is genuinely available.
That creates a real benchmark for the internal decision.
It may confirm that the internal person is strong. It may show a material gap. Either result is useful.
Compare risk properly
External appointments carry obvious risks: cultural fit, learning curve, relocation, notice periods and whether somebody can reproduce past success in a different environment.
Internal appointments have risks too: limited exposure, relationships changing overnight, stepping up in scale and the possibility that continuity is exactly what the organisation doesn't need.
Put both sets of risks on the table rather than treating one route as inherently safer.
Make the decision you can explain
For a senior appointment, I think the Board should be able to explain why the chosen person was the strongest option against a clear requirement and credible alternatives.
That's particularly important when the preferred candidate is internal. Independent assessment and external benchmarking can give a nominations committee much more confidence than a process designed after the answer has effectively been decided.
Promote internally when the evidence supports it. Search externally when it doesn't.
The useful work is getting good enough evidence to know the difference.