I don't see anything wrong with the majority of asynchornous communication methods. Mind you, I'm a fan of slack and a lot of people aren't. The issue though, I think, and the reason why slack is so widely used despite the fact that most users tend not to like it is because of the UI. It's friendly and easy, for surface level tasks, and that's far more important to people than the fact that it's synchronous (based off of my own experience, can't cite any stats so I'm not making a claim here). Email could very well work for this, and several apps have tried to "IM-ify" email, but none have really caught on. It's a UI and UX issue fundamentally, not a capability issue, once again according to my experience.