China was already sending troops and material to the front lines when MacArthur was ordered to stand down. Pushing further would have meant a hot war with China.
There is no way we could match them in numbers on the ground. Such a conflict would have inevitably led to us nuking them as a result. Which is probably the reason decision makers chose not to.
And maybe that's really the humanitarian failure. That USA didn't nuke China in 1950 or 1951. Would have solved a lot of problems for generations of people.
USA dropping nukes would have prevented the convention against using nukes in wars from being started. I think there's a pretty good chance we wouldn't have any civilization left by now if we went down that fork in history.
Maybe the real humanitarian failure is that the US didn't nuke everybody and start over from the stone age. Can't any societal problems if no societies exist, right?