I somewhat disagree with the background data this article cites as reasons for why everyone wants email dead. I do not believe people like Slack, Asana, et. al. are trying to kill email, rather, I believe they are trying to offer services that help to reduce email's dominance over communications.
One thing that email does not provide for anyone is choice. The barrier to entry is running your own IMAP, SMTP, and possibly spam blocker servers, which almost no one wants to or even knows how to do anymore. It's also somewhat inefficient for conversations, just like regular snail mail is inefficient for conversations. You mail letters, not one-line replies.
So I tend to think of IMs, chat rooms, and other methods of communication on the Internet as simply compliments to email, not replacements for it. Nothing will replace it, because nothing needs to. It's email.
(sidenote: I'm pretty sure one big reason everyone uses email is because they were given an email account, and didn't have to search out the technology. You don't see people clamoring about the reduced use of Usenet in comparison to forums nowadays. Same can be said for texting, I personally got a phone one day and someone sent me a text message, that's how I started texting. Probably never would have thought getting text messages sent to a phone that I can just talk on would have been a useful enough communications platform to seek out and possibly pay for.)