These must be paid shills. While it's actually quite understandable why real people and businesses would want to use and recommend MS Office, no one in their right mind actually thinks Teams is the best video chat tool in the world. Any serious business uses Zoom, Slack, etc.
Biggest one for me was that I could
A) start multiple chats with same audience and rename them - so I can have chat with thom dick and Harry on system architecture over a few days and separate conversation with them on performance testing issues. This is trivial in teams. I have to create awkward channels in slack to approximate the functionality.
B) seamlessly start a conversation with two people, then as you troubleshoot and expand, add more people, then jump in a call, then finish a call and keep chatting. Until recently slack would force you to start a new blank conversation when you added people - absolutely useless. Now they've hacked a solution that works up to arbitrary number of ten people and is so clearly a script in the background which still creates a new chat with added person but helpfully copies all conversation over. Then you need to add 11th person and too bad you've hit the magic number.
In operations setting and evolving incidents, teams was just better. And don't get me started on slack "huddles"!
My inlression has always been the opposite - startups used slack because it was cool. Serious businesses used teams because it worked and integrated well.
Now. I've realized lately that when people talk about slack vs teams, they're usually not actuslly talking about slack vs teams. They're actually talking about their companies security and usage policies, as incidentally instantiated through the collaboration tool of choice. I've become aware that my experience with teams is bit everybody's, due to various policies and limitations imposed, and similarly for slack.
But mostly... Not nearly as many people that disagree with average internet forum dweller are paid shills as may be believed :=)
As with the IBM model, I imagine it's simpler for companies to have a single source and a single support channel. It is possible to use Exchange sign on for non-MS systems and apps however.
Isn't that the problem with Teams? Instead focusing on highly usable text chat, the focus is growing the pile of integrations with other mediocre Microsoft products.
(Not that Slack is great; it's been bloated and slow for a long time and has likely been on steep downward trajectory since the buyout by Salesforce.)