Ion thruster and chemical rockets are for different purposes. I don't think they'll ever replace each other for the roles they are designed for.

Ion thrusters are good for control when in space (like controlling the movement of a satellite) because it doesn't need large solid fuel reserves to operate, but it's also not strong enough to push through the atmosphere.

Chemical rockets are powerful enough to push through air, but they take too much fuel and burn too uncontrolled to be useful for control once in space.