Wasn't a downside to IRC so much as there are UI/UX stuff in Slack that helps a lot. These are small things that seems obvious now, but they help with communication.
I've been working on applying the theory of Kanban starting from first principle: "Make work visible." Visibility helps develop trust. Slacks helps with signaling a lot, and there are obviously a lot of thought put into the design.
Some examples:
1. URL for every message. I can paste that URL into Github PRs or Jira to reference a conversation where people talk through something.
2. Cross-device notifications and fallbacks, and other controls such as Do Not Disturb, etc. This allows me to control heads-down coding vs. when people needs help.
3. Notification integration helps. I know there are IRC bots that can do that, but how well you can just click through things depends upon the IRC client you choose. Being able to see a log of say, Github and Jira (usually in a separate channel) helps stay in the loop. The same with when adding CI/CD notifications.
4. Starring messages helps me manage time. I don't read everything people send over the chat. I certainly do NOT try replacing person-to-person contact with Slack (or IRC or HipChat). Sometimes I'll star something I want to read later (which may or may not happen).
5. URLs posted often have previews, which also helps with managing whether I go read it or not.
I'm sure these things can be done with IRC, and Hipchat probably copied over some of these ideas. Mattermost has certainly cloned the important stuff. By no means am I saying any of this seem earth-shattering innovative, but it is clear to me that Slack was designed around remote working, and it works well.
To keep things in perspective too: before Gmail, I used to run my own web server and used terminal-based apps for that. I don't do that anymore because it was easier to just dump things in Gmail and then search for things I need. I'm still keeping my tmux+emacs setup though -- maybe I'm missing out with web-based development.
So yeah, Slack is a great tool, and it works well with what I do. If you don't use it with discipline, or you have not developed much of a philosophy on how this all works together, it's not going to work well. You won't have a philosophical framework in which to curate or vet what you want in your life or attention-space.