In the older, better days of the internet assholes would be banned frequently. The problem is that in recent years instead of having a large number of relatively small forums we have a few massive social media sites that effectively control communication over the Internet, and being banned from one of those would be a very big deal.

With the Fediverse, we can go back to the way things were, where banning someone from a given instance isn't a huge deal since people can just make accounts on other instances, but it's still enough of an inconvenience to act as a deterrent. And if they keep being an asshole on their new instance then they'll get banned again until the only instances that'll take them are ones that cater exclusively to assholes, and those can be defederated.