The tree format seems an advantage, if anything. It naturally separates discussions into separate threats. Messaging software would dump all these into a single channel so you could have different conversations happening at the same time interspersed.
Agreed, a single thread is painful if it’s actually spawning off multiple sub-topics. I suppose the better answer is to start a separate thread in Slack in that case but it can flow weirdly where the topic originally arises in one place but is continued elsewhere; it relies on someone linking on the original thread to keep context. In a mailing tree, that context is still there.
All of this depends on having a sane email client though, doing it via outlook or gmail is a nightmare and I suspect this is the root of many people’s aversion to email.