Yeah, I've never seen any value in Twitter. They should've called it "Public IM" or limited IIRC and make it function well like IM. Even the poor implementation of threads in Slack is way better than the perverted version of Twitter.
Apart from being stuck to a single level in depth (which IMHO kind of makes sense in the context of an IM platform), what's that bad about Slack's threads?
I am not arguing that Twitter is the perfect interface for its content, or even that its content is particularly good. I am just saying that it can have value regardless of its (many) faults.
Not sure what would you have preferred? I can only think of inline, but I'm not sure how well that would have worked in an IM application, where you'd typically expect things to be in chronological order. You basically described the point of having threads in the first place, and I also cannot think of many other existing implementations, so I'm curious.