Chat is a complement to email, not a replacement. Over time some people may take their formerly-email activities to chat, but no cutover is required.
Running two chat systems side by side, on the other hand, is a huge pain. There really is a pressure to get everything migrated and the old one turned off.
Depends on the company. At our 6 year old startup, email is dead except for external communication. Everything is on slack including ops, stand ups and informal communication. At Google, hangouts was lightly used but everything was on email.
Yup. I can't think of last email that was sent within my startup. Only clients and partners. Though some partners we have shared slack rooms, so even that may be going down.
Email is for outside the boundaries (outside-of-company, outside-of-division in a silo'd company), chat is for inside - there's still people who do prefer email especially in partner facing roles, but I think its fair to say that most prefer using chat if they had a choice.
Not necessarily true. For example the enterprise I work in doesn't allow saving chat history. This makes email better for technical discussion that needs recorded.
