Able:

MacArthur’s fixation on the Inchon landings was justified in my mind, US and South Korean forces had been reeling back in a string of defeats and they needed a big move to change the tempo of the war.  MacArthur realized he did not have the resources or manpower at his disposal at the time to conduct a WW2 style linear offensive up the length of the Peninsula from south to north.  By landing less than 20 miles from Seoul far from the bulk of North Korean forces MacArthur accomplished multiple objectives.  First, the symbolic victory of recapturing the South Korean capital served to greatly improve US and South Korean forces morale and belief that this war could in fact be won.  Second by landing in Inchon and pushing to Seoul MacArthur greatly disrupted the DPRK’s ability to resupply and sustain their more forward elements.  In short he cut them off.  Finally MacArthur seized the initiative and forced the North Koreans to finally go on the defensive.

Had the landings at Inchon gone poorly the costs indeed would have been catastrophic and the war likely would have been far more costly and prolonged.  However MacArthur made a call and he went with it, his decisiveness I believe greatly impacted the war in several ways that cannot be overlooked in hindsight.