Not all internet discussions are like that, and in-person discussions sometimes are like that. Long letters were always very rare; a prolific letter-writer in the 1800s might have written hundreds of them in a lifetime, but a more typical number might be ten. I think the majority of human discourse throughout the millennia has consisted of things like schoolgirl gossip, parents scolding their children, "ain't-it-awful" discussions in bars, and requests for phone numbers shouted over loud music in dance clubs. What you're observing is that more of that stuff is being done in writing instead of by voice.

People do seem to be more willing to disagree with each other on the internet, but that doesn't necessarily worsen the quality of the discourse.