I doubt that the majority of people who use IRC are ever gonna switch to Slack. I personally cannot stand Slack - so noisy, so distracting - if I have to use something other than IRC, it will be HipChat, Gitter [0], or Kato [1], but I guess there's a demographic that just loves wasting time with animated gifs...

[0] https://gitter.im/

[1] https://kato.im/