Why are ion thrusters often designed to operate below their theoretical maximum power level?

To mitigate component wear from sputtering and increase engine lifespan

Although running an ion thruster at its theoretical maximum power would provide the highest achievable $\Delta V$ in the shortest time, doing so accelerates the erosion of critical components like the accelerator grids due to increased sputtering rates. To ensure the thruster survives multi-year missions, engineers must often intentionally throttle the engine down to a lower current density or power setting. This deliberate throttling sacrifices immediate performance—meaning lower thrust or a lower total achievable $\Delta V$ over the mission—in exchange for increasing the operational lifespan of the physical hardware, which is essential for missions requiring continuous thrust for years.