> Rather than having flow and concentration interrupted by incoming message notifications, with email I can easily decide when to fetch and process messages.

But emails also notify and therefore interrupt. If you want to turn notifications off in your email or only poll new mails when you choose you can also mute notifications (or turn on dnd) or close the chat app.

Email sucks for chat like communication. It is great for long detailed messages. Having both is the best of both worlds.

I have work email and personal email. I have work chat (Slack) and personal chat (WhatsApp with friends, Keybase with my partner). Choosing which chat app is also a great tool for making sure I am dealing with the right audience. I don't want to accidentally message my boss about stuff I send to my partner.