Lync, was the that the app whose Mac client I've never seen run without crashing shortly after startup, on multiple machines across multiple companies? That's right, I've never successfully used the Mac Lync client to send a message, despite having it installed on probably three or four machines over time.
That's not to say that Slack doesn't have a lot to worry about. But competition from Lync, or whatever the hell they call it this week, is not one of those things.
As one who was previously employed by Microsoft, I have to believe that they would not let something that broken out the door, and that I've just had a run of bad luck of broken configurations or server-side operations folk that didn't know what they were doing. But the fact remains that I've never sent a message using the Lync Mac client, and not for lack of trying.
That's not to say that Slack doesn't have a lot to worry about. But competition from Lync, or whatever the hell they call it this week, is not one of those things.