For example, this is why Microsoft was sued for antitrust in 2001. Microsoft was alleged to have used its market power in OEM operating systems (Windows) to sell its browser (Internet Explorer) through bundling. Microsoft settled with the DOJ.
You could allege that in this case, Microsoft is doing the same thing — using market power in office software to sell unrelated chat software. This practice is harming competitors to such a degree that they seek to be acquired by other companies. It's also telling that HN regards Slack as a better product than Teams. The resulting decrease of competition is detrimental to the consumer.
Maybe you want to argue that Teams is being provided for free, so this is better for the consumer. While that nominally might be true, Microsoft must pay for development on Teams somehow. Consumers are likely paying higher prices than if Teams was not being developed. And even if they aren't, we still should consider how bundling can lead to consolidation.
Not to mention companies like Uber using massive amounts of VC money to break laws and social contracts in whole countries despite no goal of actually being profitable. Medium did the same when providing free access and then turning on the paywall after that. There are many more examples.