I have been flying nearly twice as long as you have and have never been taken off a plane, accosted by airport security in an attempt to force me off a plane or witnessed it done to another passenger. It is such an absurd policy it’s difficult to process that it exists. The proof of this absurdity is the self inflicted harm it has caused the airline since the incident. Facebook is covered with posts of those videos.
Whatever the reason United wanted those four seats it was, in fact, their problem. The violent way they chose to solve it was totally unacceptable.
It has already been suggested by others that the financial incentives should have been increased until they got takers. Apparently such a civilized concept has never been discussed among United decision makers. Something tells me they’ll be discussing it plenty now.