Slack is not replacing IRC or XMPP at all. Slack uses these protocols as part of a groupware suite. As discussed elsewhere in the thread, IRC-at-companies draws mixed reactions. XMPP is a protocol that has seen very wide adoption in the enterprise, with many implementations from a variety of vendors, with a variety of resource consumption issues. XMPP is very much a web technology in the sense you're talking about.
And you are correct that it proved to be a very bad stack for something like Slack, when Google first tried it in 2009: https://en.wikipedia.org/wiki/Apache_Wave
If you really think Slack has made poor technology choices, I'd suggest reading what Stewart had to say on the subject in this interview: http://www.gamasutra.com/view/news/182287/The_story_of_Glitc...
This differs for everyone. For me, it merely replaces XMPP/IRC: group-chat. That is in the last 3 teams where we used slack.
Others, and I guess mostly people who live in their email-inbox, may see Slack as a replacement for their Mail Suite.
Again others may see it as a replacement for teleconferencing/skype.
It really depends where you come from. Me, I come from a simple, integrated IRC and XMPP client. Now we all use Slack and I have a poor experience compared to the Just Works[tm] chat (through empathy) on Ubuntu.
* Slack crashes 2, 3 times per week. Mostly memory issues. Empathy never crashed on me, that I can remember.
* Empathy is, AFAIK always on. I switch off Slack when not working because (1) it abuses resources and (2) it addds another icon to my toolbar (empathy integrates in Ubuntu's message icon).
* Empathy starts whenever I start my OS. Slack can be configured to do so. But when it does, its slow startup time and CPU-gobbling while starting makes my desktop appear sluggish.
I'm comparing it to a well integrated, thin and snappy XMPP client, which is what Slack replaces for me. And Slack comes out poor. All over.