I'm not sure, but why would they want to? It's a very mature product, with many established competing manufacturers, and not much room for any real improvement or profit margin.
If there were already half a dozen Slacks in use and they were all interchangable and customers were indifferent to which one was on their computer, would Microsoft be jumping in with their own product?
Microsoft is allowed to make more than one product. Microsoft is not allowed to use abuse their market dominance in one product to help them capture another market, with harm to the consumer.
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.
If Slack is marketing their product for businesses and as an office product ("we're trying to kill email"), then it's hardly unrelated. If companies are looking for a solution that streamlines the functionality of SharePoint, Teleconferencing, and collaboration, it seems directly within Microsoft's wheelhouse to include Teams as a part of M365.
Sure you can, but treat them as a standalone product. Currently companies (like Amazon with AWS) are using massively profitable products to dump prices in totally unrelated industries and kill off the competition.
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.
