Slack is obviously not killing emails as they are advertising through lot of PR. They have 150K active users so far. However one thing that they are correctly going after is fantastic search abilities for our own data that would include all emails, chat, attachments etc. My feeling is that eventually communication client that excels in search would indeed surface to the top. This client would then drive communication standards of the future, including emails. So in essence, problem is not integration of desperate sources of communication, but ability to search them efficiently.