I think usually the argument is that they're leveraging an _existing_ foothold to gain advantage in a _new_ market. So the existing foothold is MS Office and MS Office 360, and the new market is messaging.

How they gained this foothold in desktop productivity software is left as an exercise to the reader.

Hey, thanks! I don't know much about antitrust, so this is likely to be a dumb question;

Why is messaging a new/different market for Microsoft regarding O365? Microsoft has been including business-targeted messaging/collab tools in O365 for years. Skype for Business preceded Slack, let alone Office Communicator or Linc, not to mention Outlook.

Is using existing products to leverage yourself into a new market anticompetitive, or "how any company expands into a new market?"

Depends on the scale of your foothold

> Slack is thriving.

Is Slack profitable?

$780m profit in 2021.