Maybe if Slack offered proper video conferencing tools it would not complain. Should ask Zoom for advice.
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.
It is hilarious that Slack still has not been able to introduce proper video calls while introducing relatively useless integrations.
I do agree with the EU that Teams should be unbundled, but unless you have a cheaper and/or better alternative I don't see it being all that effective. Teams also integrate into Active Directory and the whole Microsoft ecosystem in a way most other systems just doesn't do (not that they can't).
And that's thing - if slack was actually trying to add features and integrations that users actually wanted, it would not fail that much.
I mean, Zoom succeeded without having anything except team conferencing...