You can't pay for Excel without also paying for Teams. Why would you pay for two team chat solutions?

Teams is not actually free for people who don't want the rest of Office.

That being said, I don't this should be illegal.

There is a free version of Teams now: https://www.microsoft.com/en-us/microsoft-365/microsoft-team...

Office web apps also have free tiers, although I don't know if there are business licensing restrictions.

Microsoft have a monopoly on office software. All businesses more or less have to buy Microsoft Office. Microsoft bundles Teams for free with it.

There is something stopping businesses from using Slack - the fact that Microsoft gives them Teams for free.

To be honest I doubt this will be a successful case. Companies get away with this all the time. I think the Windows/IE case is the only instance I've ever heard of something coming from it.