Microsoft's entire business model for decades has been to shove shoddy products down people's throats. And somehow, they have figured out how to do it too, because otherwise Teams wouldn't be used by anyone.
Microsoft’s best pitch (and Google benefits from this too) is that contracts are annoying and take forever to execute. If you can sign a deal for Outlook and Teams it’s so much easier than separate contracts for Outlook and Slack. You’ll get very far with that logic alone.
Most companies I’ve been at that use Teams over Slack is not “We can’t get contract for Slack” but “We have Teams included, why would we pay for Slack?” - Accountant
I guess Microsoft lost this battle, at least at some companies, because I'm now at one that uses Slack and Google, with no dependency on Microsoft Office.