"no airplane or missile can ever take or hold ground. Only the Soldier (or Marine) with a rifle can do that."
But any piece of ground may be rendered unholdable [and uninhabitable for years to come] by the enemy. And an airplane or a missile could do it much more effectively than a Soldier or a Marine.
But any piece of ground may be rendered unholdable [and uninhabitable for years to come] by the enemy. Yes, but not on a large scale. For all practical purposes, you can't make an entire country, or even a province uninhabitable.
And that's the point. If you are going to do regime change, you have to take effective control over a country long enough to hand it over to the new regime. Airpower alone cannot accomplish this. It helps, but at the end of the day, you need boots on the ground.