The actual value is complex. The only reason an engineer’s presence at the company generates $X in revenue is because the company already has sales, marketing, finance, legal, and all the other necessary functions covered. On their own, that engineer’s output is worth less. So a light switch test does not tell the whole story. The surplus from the combined output is deserved by everyone whose input is required for those $X, not just the engineer.

In different terms, maybe you leaving costs the company $X. But if product engineer Joe Bob left first, maybe you leaving suddenly only costs the company $Y, where $X > $Y. Are you really worth $X?