one generous approach may be to steer that appetite for complexity towards tooling rather than the actual system. it's not such a problem if they over-engineer

one generous approach may be to steer that appetite for complexity towards tooling rather than the actual system. it's not such a problem if they over-engineer too-smart tools to help maintain and administer your carefully managed, just-smart-enough system. When they move on, a 'normal' replacement doesn't necessarily have to reason about the complexity: if the tools break, or are too hard to understand, the next dev can revert to simpler tools. it's great to keep really smart people around, but it's also great to have a business that is insulated from risk by minimising unnecessary complexity and engineering for maintainability. if they're too smart to realise that, maybe they'd be happier elsewhere.