Same with "The Corbomite Maneuver," "The Immunity Syndrome" and "The Tholian Web." But in fairness those are fifty minute episodes where time is effectively compressed to create a heightened sense of tension. If any of those were stretched to two hours you might start running into problems. What do you add into that extra hour? This is where the comparison to "The Changeling" is apt. "The Changeling" is also all aboard ship (indeed more so than TMP), but the story is tightly told within fifty minutes. There's no extra screen time to fill.