Email started "dying" long before GMail even was a thing. AOL and ICQ startet it, Twitter and Facebook continued it and now it ends with Whatsapp, Slack and Discord. And I doubt that Whatsapp/Facebook/Discord are significant faster than Gmail or Slack, so this can't be the reason.

E-Mail is just a transport-protocol, poor in ability compared do other solutions and rather complicated, especially when it should be made safe.