Build a succession plan for the one role that would actually hurt if the person left tomorrow

Build a succession plan for one specific role that would create real disruption if the current person left with normal notice — not a company-wide succession exercise, just this one role, treated seriously because the risk is real.

ROLE AND CURRENT PERSON
Head of Infrastructure, currently held by one person for the last 6 years.

WHY THIS ROLE IS A SINGLE POINT OF FAILURE
He is the only person who fully understands the legacy on-prem-to-cloud hybrid setup that half the product still runs on.

INTERNAL CANDIDATES TO CONSIDER
Two senior infrastructure engineers, both 2-3 years tenure, neither has managed people or owned architecture decisions independently yet.

CURRENT FLIGHT RISK SIGNAL, IF ANY
He mentioned in his last 1:1 that he's been approached by a recruiter and is 'thinking about what's next' in general terms.

For each internal candidate given, assess readiness against the role's actual requirements on a realistic timeline (ready now, ready in 12 months with specific development, ready in 24+ months or unlikely) — do not default every candidate to a hopeful "could grow into it" without naming the specific gap and what closing it would require. If no internal candidate is genuinely close, say so plainly rather than stretching a weak internal case to avoid recommending an external search, since a succession plan that pretends readiness exists when it doesn't leaves the organization exposed at the exact moment it matters.

For the single most viable candidate, write a concrete 12-month development plan tied to the specific gap identified, not a generic "more leadership exposure" recommendation — name the actual project, stretch assignment, or exposure opportunity that would close that specific gap.

Separately, address the interim coverage question directly: if the current person left with only standard notice today, name who would cover the most critical parts of the role in the gap, even if that coverage is partial or comes with an explicit tradeoff (something else that would have to slip).

If a flight-risk signal was given, treat the timeline in this plan as more urgent accordingly, and say so explicitly rather than treating this as a routine, unhurried planning exercise.

OUTPUT FORMAT
1. Readiness assessment per candidate (ready now / 12 months / 24+ months or unlikely), with the specific gap named for each.
2. If no strong internal candidate exists, a plain statement recommending an external search be started in parallel.
3. A 12-month development plan for the most viable candidate.
4. An interim coverage plan for a sudden departure, naming who covers what and what tradeoff that creates.