The business/work version of Teams isn’t free, you’re paying for it as part of the 365 subscription.
That’s part of why there’s a push to use Teams: companies see it as already paying for it, so might as well use it.
And of course there’s the constant barrage of fear mongering coming out of the security crowd that says the only sensible, secure thing to do is bring absolutely every fucking thing into a native 365 app. Because they fail to notice that the attack surface they’re so concerned about is a healthy software market.