First "but it doesn't scale". Now "but integrations".
There will always be another "but", and there will always be a link on the internet showing that it's been done already.
"but shiny web gui" - https://thelounge.chat/
"but mobile app" - https://github.com/MCMrARM/revolution-irc
"but it's complicated" - https://www.mirc.com/ since 1995
IRC does have limits. Some. One is that to make it "modern" it needs a bouncer - that said znc is spectacularly simple to set up with it's web interface.
Integrations were never one of those limits. Just google IRC bots.
That's my entire point. It's death by a thousand cuts. Now I'm responsible for an IRC server, a web frontend, a mobile app and a bouncer.
The OP said "just run an IRC server". There is no "Slack or just an IRC server". There's "Slack or a number of connected, interdependent services you need to maintain". I'm not saying it can't be done, or that it isn't the right answer for some people, but it's also a lot more work (and thus, expense) than just paying for Slack. It absolutely makes sense that people do it.
When did this madness start?
Paying experts in their field (chat tools in this case) is most of the time the most reliable and cheapest variant. You must also include that Slack has plenty of features that are not available in IRC and mean the productivity is not as good.
30 years ago, efnet was (apparently) 35k users.
I think it's really sad how the world has migrated away from an open protocol to a closed one, but I remember IRC having some very real annoyances, well beyond stuff like integrations.