Not sure about that. Though I agree with you to some degree about the level of violence that occurred.
That said, the couch was too badly damaged to drive back to Manchester. Indeed if the coach had stopped I think there could have been serious problems that the Liverpool police would clearly not have been able to handle.
The sad thing is the scum got away it and will thus continue to do it. It could have been stopped in its tracks by calling the game off and giving City a 3-0 win, but no, the powers that be are happy with the result. Indeed it resulted in a copy cat action a few weeks later in the French league.