In an interview with The Verge, Butterfield said: "Microsoft is perhaps unhealthily preoccupied with killing us, and Teams is the vehicle to do that."
Butterfield personally thinks that Vole Teams is more of a rival to Zoom. While Slack has its own voice and video calling features, it's not the primary focus of the app, and often, businesses integrate Zoom or Cisco's WebEx instead.
Microsoft has been moving businesses from Skype for Business to Teams, which traditionally focused on voice and video calling. Ultimately, Butterfield thinks Microsoft is trying to force the Teams comparison because "Microsoft benefits from the narrative that Teams is very competitive with Slack. Even though the reality is it's principally a voice and video calling service".