I don't know why I need to use slack [...] just
* User authentication
* Support for multiple concurrent logins by one user
* Persistent, searchable history
* (Ad-free) file and image sharing built in
* Simple integrations, like webhooks, built in.
In other words, Slack is like IRC+NickServ+Irssi+Screen+Imgur, except easier to use, in the sense that you don't need to know key combos like Ctrl+A+D or Ctrl+Alt+2, you don't have to figure out how to send such combos from your phone's terminal emulator, and you don't need access to an always-on server to run your screen session.
Of course, it's not all good; Slack has a bunch of opinionated design choices, like a channel it's impossible to leave, no ability to block users, no off-the-record option, and suchlike.
They even give you a hello world sample curl when you opt to add the webhook. At the simplest you can just replace the hello world text and bam -- you're sending to slack. Just takes a very simple json input.
chances it will be around in 10 years? I would say 25%.
 - https://github.com/mumble-voip/mumble
 - https://wiki.mumble.info/wiki/Main_Page
Also now that Discord exists, I would never do a voice chat in Skype either. A substantial portion of every Skype call I've ever been on was people apologizing to each other for the bad audio. Discord apparently just has better signal processing.
Skype for Business does. But... not the Azure/Cloud version; you have to host it on-site, and MS are rapidly replacing Skype with the less feature rich (if that's even possible!) 'MS Teams'.
None of the chat apps ticks all boxes, which is why we need a universal client that puts the user back in control like in the Trillian/Adium days. And no, matrix+bridges is not that solution.