I don't get that, really. For at least a couple of decades there has generally been some internal instant messaging system at whichever company I've worked for

I don't get that, really. For at least a couple of decades there has generally been some internal instant messaging system at whichever company I've worked for. Nerds used to like plain local IRC server. In some places the expected tool was Windows Messenger despite it sucked bad and kept disconnecting. Slack is just another communication system: if you have problems with Slack you're likely to have problems with any messaging system if you configure them to allow interrupting your work. The very reason I've personally always preferred online communications for anything non-urgent at work is because nobody gets to interrupt me and I get to check messages when I have an idle moment. The alternative is face-to-face communication which means someone is interrupting you at your desk several times a day which I ten times worse. I currently use Slack over their irc gateway which works just fine. I have an Slack-connected irc client running in one tmux window and when I have time I can easily see if someone called for me or if the team channel has new activity.