Offloads happen for a variety of reasons, day in day out. I would hope those contributing to the discussion are well familiar with the concept.
United's real problem is the execution, and raises a lot of questions. If we assume it was a last minute requirement (rather than a stuff up at the gate), how do you proceed with the offload when an adult refuses to budge like a grumpy toddler?
The outcome would suggest, random selection doesn't work. Perhaps they should they should have sweetened the deal until volunteers emerged.