Earlier quoted context omitted.
Slack's benefits over IRC are largely not subjective: * The ability to directly send files and images to channels is an objective benefit Slack has over IRC, which has even less ability to do that than MIME email (at least MIME lets you post the base64'd file contents of your image, rather than just a clickable link). * The ability to send long messages or whole source files is an objective benefit Slack has over IRC…
You've been able to directly send files over DCC via IRC clients since like 1995. Interpreting and displaying those DCC'ed files or cutting longer messages into sub messages and recombining them is something that you could implement via the IRC client just as easily as it was implemented in Slack (not to say that it would be easy, it'd be a lot of work, but it likely would not be much more work than what took to make…
Given the problem of sharing a diagram with a team of developers on a group message channel, I'm sure there's someone here who would say they prefer the DCC option of receiving a named file and then opening it with a file viewer, rather than simply dragging the file into the chat window and having it appear instantly to everyone on the channel. The existence of that person is not interesting to me.