The other passengers on the lane SHOULD have stood in the aisles blocking the removal of the passenger being forcibly removed. Civil disobedience has good effects against irrational authority.
I would have stood up and blocked the aisle. I'm glad it was videoed, but that wasn't sufficient action.
Obstructing a law enforcement officer is a crime. And you'd probably get a taste of that passenger's experience yourself.
Imo, the passengers took the best option available to them - they got video of the event with them clearly expressing their disapproval and made it a viral hit. If they hadn't whipped out their phones when they did, we wouldn't even know this was happening and United Airlines (and airlines' overbooking policies in general) wouldn't be suffering a public backlash.
They did good. Not everyone can be like Superman, but a lot of the time, being like Clark Kent is enough to do good.