What you've described is just as applicable to nearly every other popular communications platform whether it's Slack, IRC, Skype, Twitter or even email.
Slack didn't invent those qualities. It's not unique in that regard.
What Slack does do better than most is provide a half decent interface for real time chat amongst groups of people. Personally I'd prefer vanilla IRC because I'd then get to chose my chat client but in terms of official clients go by, Slack is better than most (though that's really more of a criticism about Skype, email, etc than a compliment to Slack. I mean how is it that so many collaborative tools have such poorly designed UIs?)
When Slack is down for a few hours, I know start-ups that feel it in the gut.
As a social tool, Slack is one among many, but social tools for work are becoming huge because they enable passive knowledge transfer. Employees can spy on others' conversations to see who knows what and that knowledge makes companies more efficient.
Something's lost and something's gained in living every day, but Slack isn't a fad and it's not going anywhere.