Slack and all its clones are based on the chat room model, which structurally has the problem described in this article (and many others for productivity, such as wasting attention). Fundamentally, the chatroom model pioneered by IRC is poor for asynchronous communication because you can't sustain temporally overlapping conversations in a channel.

However, you can't "just use email" -- Email's threading model is great for asynchronous work, but it is poor for synchronous communication and also doesn't support modern features that make it easier to communicate ideas efficiently (shared history, markdown formatting, image previews, emoji reactions, etc.). It's essential to be able to have (semi)synchronous written conversations with people you're working with, especially if you don't want to spend your days burning out on video calls.

This is why we created Zulip -- it's designed as a real-time communication tool with email's threading model and all the nice features of modern chat apps that email lacks. And the reading user experience is actually a lot better than email, because Zulip provides is designed with the goal of saving time when prioritizing, skimming, reading, and replying to conversations. It's also 100% open source software (not open core!).