if that solitary Star Destroyer could actually fly around the planet at will inside the atmosphere, or from orbit in space, a lot of the pro-Imperial arguments would have more justification.

then they really COULD go where they choose, strike where the wish, and dictate the terms of battle. they could actually barter their military services in exchange for supplies and a place to stay.

based on the declared limitations of the OP... I think it's pretty clear that they CAN'T do this. they can't fight battles of their own choosing, and fight them on their own terms. they would be stuck in a single spot, and eventually forced to fight on terms favorable to the Earth's ground forces.