Earlier quoted context omitted.
Teams is the result of Microsoft taking a look at Slack and saying, how can we make this feel more like Excel?
I don't see where is that a problem? People who will be using Teams are people using Office 365 daily. Having a cohesive ecosystem makes complete sense. From a UX too.
I'm old enough to remember when Skype was really neat, and it's just another in a long, long line of grievances I'll hold against Microsoft till I die.