That's effectively what I do as well, but we don't have a corporate Slack account.

Teams is great for internal communications on long-running projects. It's well suited to that and I wouldn't substitute Slack. When used well, it cuts down on a ton of internal emails and status updates.

I don't really blame IT for locking down Teams. It makes a lot of sense for all the reasons you mention and many more. It's just that by locking it down they prevent Teams from really being a challenger to Slack.

It's not that Teams can't be configured to be just as useful as Slack, but enterprise IT best practices pretty much guarantees that it never will be.