On the other hand, what knowledge worker today can function without video conferencing, desktop sharing, file sharing and VoIP?
No that many.
Teams provide all of that natively. That's why they're wining.
Let's be frank, if Teams had been just a chat application, it would never have had the success it currently has, even as part of O365. Being bundled with something popular is not a guaranty of success. IE is a very good example of that.
The reality is that MS made Teams a complete collaboration platform and Slack couldn't do the same with their product.