You are right about a lot in your response but what you missed, and why SLACK (and not IRC) are used in organizations more readily, is the following:
- Ease of use. No arcane IRC lingo to know, No terminal client to configure, just point and click. (example: https://en.wikipedia.org/wiki/List_of_Internet_Relay_Chat_commands)
- Paste anything. Graphics and spreadsheets show up right in the thread.
- Cross communication. Different groups seeing posts from others outside their silo without having to forward an email or forgetting to CC that individual. (this is obviously true of IRC as well)
- Support for every platform. Android, iPhone, Windows phone, PC, Mac, Linux and, of course, web.
IRC it is, but a tad more as well.