Communicate the metrics you value for long term resilience and that efficiency is not more important. Explain why, use the example of them getting into a car accident.

If you have to fire such a person you failed at communicating the relevant metrics to them. They are clearly going out of their way to optimize for something, they are simply working with an incomplete understanding of what to optimize for and what costs that incurs.

edit: Communicate it like you would communicate that the great new improvement in start-up time came at the cost of energy consumption or unit price. Its great but it missed an existential metric.

While there are people that maneuver themself into positions making themselves irreplaceable, quite a few simply have the urge to clean up and make a nicer solution. Their definition of nice is just incomplete.