XMPP/Jabber has always supported offline message storage.
I don't hate Slack; heck, I've never even used it. But like _many_ people I'm disappointed that we still don't have a non-proprietary, federated IM system. That leaves most of us ambivalent, and some of us bitter.
IRC is still around, but IRC isn't a substitute for IM. IRC can't be truly federated because there's no global directory or routing the way there is for SMS or XMPP.
XMPP is still around. I can even chat with a few GTalk stragglers, and quite a few tech-savvy people still run their own XMPP servers. But it seems to be on the decline. For a brief period it was gaining corporate momentum, but that stopped years ago.
My company still uses XMPP for corporate IM. Fortunately, nobody can agree whether to move to IRC, HipChat, or Slack. So for now I can actually instant message with co-workers from my personal XMPP server/account. That was actually useful when I was on contract and didn't have (or want to deal with) juggling and logging onto corporate accounts.
Somebody else suggested Slack was like the iPhone, in that it offered features and usability previously unimagined. I don't think so. The iPhone wouldn't have gotten anywhere if it wasn't a _phone_. It would have just been a fancy minicomputer if it didn't have the built-in capability to interoperate with any other phone, and participate in the global federated telephone and SMS network.
For all of Slacks features, they pale in comparison to what _could_ be. They could still create a better IM client, with better searching, indexing, etc, etc. But if it supported global federation a la XMPP, the sky would be the limit. Instead of the Internet people are simply excited about an upgraded AOL experience.