The main, really only, features that Slack has that IRC doesn’t is easy account onboarding and (crucially, for business) offboarding, and persistent messaging. This has been discussed to death, publicly, by organizations like Mozilla. They were also smart enough to offer themselves for free to startups, which largely undermined hipchat, which had the same basic value proposition over IRC, and allowed them to build a hip reputation and network effects in the startup crowd.
Are we really going to pretend that those features are why the Slack client consumes tons of RAM and a fuckload of CPU cycles? They’re entirely server side, for christ’s sake.
ICQ could do all of the above on a 32 Meg Mac 20 years ago. The bloat isn’t related to the reasons it “won”, or why it continues to be popular. Network effects, the fact that if your company mandates it you have no alternative, simply insulates them from suffering many ill-effects of their bloated, laggy client software.