Off topic, but this astounds me.
Not trying to be elitist (I'm genuinely curious), but what is it about threads that throws them off?
They seem unrelated to software. I would have thought "non-tech" people would also understand them.
One guess is that there's the mental effort in keeping track of these threads (even when they're literally right in front of you in the interface.
Maybe it's the fact that they're just so used to the whole email chain with 15 levels of quoting old emails that is so common in many companies. Many many people are mentally immobile and unprepared to switch to anything else, once they're used to something. Lifelong learning is not something that is common in every job, like it is in software development.
Again you can't really ask me because I find threading totally awesome.
Personally though I have nothing to complain about with slack. It has threading. It only has one level threading but that might actually be a good thing because we slack with non tech people too. One thread is sometimes hard for them to grasp. Make that a regular threading model and they will utterly get lost. You can see how that completely breaks down with emails in most places.
We use slack both for synchronous and asynchronous communication. It's all about the culture and what people expect. Not really different from email. Remember the people that send you another email if you didn't answer their first one after 5 minutes?
Group chats you can just link to in a ticket instead of having to paste and reformat from a large email conversation? Priceless!
Markdown is not necessarily required. Most good email clients supported something like it, like actually displaying something like _this_ as italics or * bolded *. It's like saying "but your app doesn't do XML". Yeah well but it does JSON.