And the crew (or gate agents) working the flight highly doubted that he would resist the police order and end up getting shit kicked out of him.
I wonder what the public support would be if, in this hypothetical case, I refused to leave the room, and refused to comply with the police when they inevitably came to remove me from the room.
Removing the irrelevant stuff (you unpacked, the new person is more important), I guess what I am getting held up with is; does it really matter whether you are already in the room or not?
Being told you can't have the room (or car, or seat, or moving truck) that you reserved sucks, I guess the emotional aspect of having something taken from you once you "have" it is worse than if you are denied it in the first place, but they shouldn't be allowed to do either.
Without penalties or laws to prohibit overbooking from happening in the first place, I can see more cases like this appearing. And, if the ruling is that once you are "planted" it is yours, I wonder how many people will go ahead and try and charge their way onto the flight.