What is "finish the job"? Iran is a large populous country that's mostly been a unified polity since Cyrus the Great. They are a different people and culture from their Arab neighbours - how will the Arabs rule and why will the Persians follow them? A colonial rule by the US will be more disastrous than the experience in Iraq or Afghanistan - in those days cheap autonomous drones didn't drop grenades on soldiers.

The present US President is smart enough to realise that now. In this case he let himself be misled by Israelis & their supporters that Iranians would rise up and replace their own Government. That indeed might have happened if the US had intervened when the Iranians were actually protesting some months before the present crisis. Now the US administration is looking for a way out without putting boots on the ground and Iran is looking to haggle on the price and for the US. this very business like cutting of ones losses is almost certainly the right move.