We used to socialize in the streets, in the marketplace, in towns. Maybe the town's butcher was known to be a bit of a racist hick, but he was still a decent shopkeeper and his kids went to the same school as yours, so you tolerated his antics. There was the town slut, the town's drunk, the guy who was a known communist sympathizer, etc. People with their flaws, and qualities, and somehow we managed to make it all work.
What happens when those people now each socialize on their own online social network with people exactly like them? That can't be good.
If we allow them space on our open platforms, without demanding that they tolerate our views, that does not result in any kind of real conversation, it just allows them to spread their toxic views around.
It's true, you usually can't engage people when they are pumped up on adrenaline shouting at each other on Twitter, but that's like jumping into the middle of a brawl and saying "those people will never be able to have a conversation" when they don't immediately stop fighting and listen to you. I've talked to people from far left to far right, from vegans to conspiracy theorists, and with very few exceptions, all of them were happy to explain their ideas and viewpoints and were pretty accepting when I expressed a different opinion. They may well believe that I'm wrong, but they didn't mind talking to me after learning where I disagree with them, and they didn't become combative either.
I believe a very important part about "open discussion" is to be honest and compassionate. Don't talk down to people, don't mock them, don't tell them they are scum, or evil, or destroying humanity etc and then say "they don't tolerate dissent" when really they just don't react kindly to hostile behavior.
I feel like this is the goal online communities would like to see of their users. Be polite, accept good faith, etc. HN tries to have it in its own guidelines.
The startup that manages to motivate users to, while disagreeing vehemently on a given argument, remain (civil|good faith|each side remembering that the other side IS human) on target without descending into flame wars or otherwise, would stand to make a TON of money.
I feel like perhaps that is an impossible ideal to pursue though.
I tend to see things in a functional way, and I often think that the people that fight on Twitter are using that as a way to blow off steam. It's not healthy as in "it will make them have a better tomorrow", but it might manage the pressure before they explode. If your life is shitty and constantly screaming isn't socially accepted, screaming silently by telling somebody to fuck off might be the next best thing, similarly to mental health issues and substance abuse.
Personally I am fine with them isolating themselves in cesspools like Voat. While they are not being exposed to contrary views there, they are also not exposing large amounts of other people to their views.
Another factor is that toxic views are massively amplified by bots (and the bad actors behind them) and not providing these with an easy-to-exploit platform is a good thing in my opinion.