A number of people have found likely alternatives in this thread. And you have hand-waved them away.
There have been a large number of people on here who are infected with liberalitis. This was a situation lacking a good solution....
No, you even admit there was a good solution - offer more compensation.
Most liberals are unable to make a sane choice in such a situation and go with the path of inaction, no matter how much worse that makes the final outcome.
Most kneejerk authoritarians would agree that using involuntary methods and violence did not make this outcome worse.
How about the following scenario:
The airline backs down. Scumbag is allowed to fly. However, every passenger who doesn't get to fly tomorrow due to the lack of crew gets a letter identifying Dr. Dao's refusal to comply with an IDB as the reason and suggests a class action lawsuit.
Now we get to the crux of the matter - it was all about not backing down. UA could have asked another passenger or even announced that the flight would be delayed until someone volunteered to leave. But they did not. Neither they nor you could fathom "not backing down".
BTW, your scenario shows you are infected with the worse case of liberalitis in the world.