That is exactly it. Hindsight. In hindsight I'm sure a lot of the people involved would have made different decisions. That's easy, and it's easy for all of us to criticize what they did, given that we have the benefit of hindsight (and a distinct lack of solid information, so the usual outrage machine has taken hold).

This was a very rare event: in the majority of cases, they're able to get volunteers to take a later flight. In almost every single one of the cases where they aren't, the people involuntarily bumped leave without incident. I doubt anyone on the flight crew foresaw their chain of actions ending in violence, right up until it happened. And I think that lack of foresight is entirely reasonable and understandable.

United (as a company) did a poor job of handling the aftermath, though.

A more likely scenario of crew being out of position (or exceeding duty/travel time, and insufficient rest) would most likely result in a cancelled flight, and that would be relatively expensive for the airline.

The cost of involuntarily bumping 4 passengers ($800+ x 4) assuming no further incident, rapidly approached any benefit from not cancelling (or delaying) the morning flight. Total revenue for the regional flight would likely be less than $10,000.