It think that one of the issues in this case. Teams rather terrible, but so is everything else.

I worked for a company that used Google Chat internally and we'd use Teams with some clients. People kept telling me how much better Slack is, but now that I work for a company that does have Slack, I can't say that it's any better. In fact I'd probably rank Google Chat highest of those three options.

Microsoft is giving people free replacement that is in some ways better (but with a worse interface). There's no point in NOT using Teams, when it's at least no much worse. In the same way where people are using Google Chat, or whatever it's called now, because they use GSuite / Google Workplace and the integration is really good. Teams just needs to be cheaper than Slack, unless Slack massively overhauls everything.