Slack is merely the latest contender in a very long succession of group chat technologies. They will be usurped one day, but they are substantially accelerating their demise by making their product so infuriatingly slow.
Microsoft does have people on staff that know how to make an Electron app not suck really horribly, though whether any of that expertise from the DevTools branch makes its way to Office and the red-haired stepchild UC teams...