Even assuming the correctness of your argument, the threat of violence only applies where it is credible. Strong bullies aren't intimidated by their victims, men generally aren't intimidated by women, and so on.

There is in fact an orthogonal effect where people will fear expression in the face of violence. Is "civilized" expression dominated by the strong better than the rudeness of online communication, where there are different power dynamics? I don't know, but I wouldn't be so quick to dismiss it.

Perahps at least a portion of the "uncivilized" speech you see online is the outcry of those who are not allowed to express their opinions elsewhere.