I'd argue that Teams is bigger than Slack because Microsoft pulled a fast one, exploited their monopoly, and included it "for free" with the rest of their licenses, rather than getting people to sign up de novo. The fact that it's worse in every meaningful way than the competition, yet far wider used, highlights the whole problem quite neatly.
We have to use teams because teams is free. Despite it being terrible. There was also a push for us to switch from Miro to Microsoft whiteboard. Until a senior manager actually had to use whiteboard and they realised it was terrible.
