I don't think drones could totally replace boots on the ground. There are places that combatants can hide without being discovered or accessed by drones. If we were to get farther into unmanned technology, like robotic soldiers, then I think we could potentially see a huge reduction of real soldiers. Of course, this works much better as offense than defense. At some point, a drone attack against America creates the necessity of those being attacked to become soldiers to defend themselves. I think that this is all quite a long ways of, if at all, though.
Drone might be able to replace boots on the ground, although you are right there certainly are locations that cannot be access with a drone. Soon the drones will have more competent firepower and this will no longer be a problem... I think the main thing that is and will hold drones back is the civilian casualty rate, if it is even released. Boots on the ground may kill a civilian or two but the soldiers will carry the emotional burden unless they are sociopaths. A drone is a machine.