Yes, but are those the only people using Slack? Most of us communicate with people for whom that's non-trivial. Even if it is, it's effort to set up, has to run somewhere, and it doesn't integrate well. It's a crappy solution to the problem, when Slack's is 'scroll up'.
In my experiences using Slack, the people who aren't the normal IRC crowd (take that as you will, heh) fall out of common use after a short period of time. You're left with roughly the IRC crowd.
That may be dependent on team dynamics.
Obviously, small sample size, but that's not my experience at all. I've used Slack in two or three different environments for 1y+, and people didn't seem to drop it.
Depends upon age. Anyone in the UK under 30-35 grew up using MSN messenger so even the least technical of non-technical people are happy using something that as far as they can see is basically the same as MSN.
I had a persistent IRC backlog pre-Slack too. It's less difficult to get your colleagues that dont read "hacker news" to do it.