The problem isn't that there are assholes on Twitter. They've always been there. Dealing with abusive people who want to shout down things they don't agree with has always been a downside to social media, especially if you share or talk about any remotely "controversial" topic. Twitter's lack of tools to help manage that problem didn't help but no amount of tools will completely fix the issue. It's a fact of life that a minority of people are just not very nice. The real problem is that there's no benefit to being on Twitter any more. The upside has vanished. The genuine conversations and fun side of Twitter has been lost. It's become a crowd of people all broadcasting without wanting to engage with their followers.

I suspect it's simply a network size problem - once Twitter hit a threshold of users it got too noisy. It's ironic that Twitter's value is considered to be the size of its audience whereas that might actually be the exact thing that's killing its usefulness.